I have the G110 with the 11 HP diesel, and I also happen to have the big 71" double action cutter bar. (I also have the biggest flail mower and the stump grinder as well). While I'll leave the hay issues to the experts, I can at least give my impression after using the equipment you are looking at. I have the foam-filled tires as well for some extra weight. I have some steep sections that I mow, particularly a bank along the road, which I'll try to measure at some point to give me a more clear idea of what 4 acres at 30* would be like.
1.
11 HP diesel engine. Can the diesel operate for long periods of time at that angle, or will it either starve on lubrication or feed oil into the cylinder and go berserk? Is gas safer at that angle?
The service manual states:
continuous service for up to 30 min.: 25*
Max. inclination discontinuous service for about 1 min.: 35*
There may or may not be a safety margin is baked into these figures, so you're kind of border line here. I suppose if you intend to use it mainly for this task, you could get a smaller tire to put upslope (or bigger on the downslope side, or a combination of both) to help level it out. Of course getting back to the starting point may be problematic, but you could walk it in reverse or flip the controls around and then use 4th gear to scoot along. I'll also point out that the cutter bar, according to the manual, is not to be used as transportation by standing on it
2.
Sickle bar. Earth Tools offers a 71 double-action bar. An 11 HP diesel could probably handle 96 with no problem. Does such a thing exist for a two-wheeled tractor? The Brielmaier has a 14 foot bar. Is that cool or what?
No idea what's out there for other alternatives, but the 11 HP diesel is overkill for the 71" bar. The biggest problem with the cutter bars is you have to keep the engine speed slow which really limits your ground speed, even in 3rd gear. Throttle needs to be around idle-1/4, so if you need to move faster you'll have to factor tire size into the speed equation.
On the plus side, this thing just sips the diesel. If it used any less fuel you'd be siphoning it out after each use! I haven't kept close track of consumption but I'd bet you could finish all 4 acres without needing to stop for fuel.
Another concern with a large bar which might not matter in your circumstances is maneuverability. The ends swing out WIIIIDE when turning, which would be considerably worse the longer the bar gets. Not an issue with a nice clean hay field, but if you're dodging metal fence T-posts and other things you
really don't want to hit (said barbed wire for instance), then it's something to think about. A 14 ft bar would be pretty awesome though.
The last thing I thought of is that I've noticed the bar likes to pull to the side that has the densest material. I've been using mine to clear overgrown fields (after going through with the flail mower a few times) and if you have a lot of variety of grasses, like thick clumps mixed in with taller fine grasses, it really beats you up after a while. And with the 71" bar, it can really want to pull you off course. With nice hay, probably not a concern, but given the dire consequences you mentioned I thought I'd point it out (I never had even thought of this until I experienced it first hand). This leads to two issues which I'll talk about later (diff lock and brakes).
3.
Wheel extensions. I am thinking the 16 would be best for a stable footing. Or is the adjustable version worth the extra expense?
4. Wheels/tires. I am thinking not just balloon tires, but double balloon tires. This would provide a light footprint and flotation in the softer ground and help with stabilization on the slope. Good idea or folly? Is it even possible?
Don't know, but good advice seems to be "buy the biggest wheels/tires you can afford". I went foam filled, but I wish I went bigger. Of course, I'm clearing rough land so I get stuck a lot and could use the extra clearance, but the extra ground speed would be nice too since I'm rarely ever at full throttle (only with the flair mower when diving into a really thick patch).
I'm pretty sure I've seen pictures of double ballons, maybe on Earth tools? I don't think you need the flotation so much as the extra treads. I would go with dual Ag tires over balloons because I think they provide more traction, but to be honest, but I don't know a lot about optimal tire choice.
5. I would use a 72 Molon rake to throw the cut hay up to the top of the dam. Conventional compact tractor equipment would be used to bale the dam hay, as well as the hay on the soft ground.
Can't help you there, sorry. Sounds expensive though if you don't already own the Molon rake. I've looked at buying hay equipment for some areas around me but it's just not worth the entry cost for what I'm using (or paying for it).
Some other random experience from my time behind the seat, err, wheel, uh, handlebars:
The hardest part of mowing sideways on a slope with this machine is that it always wants to skew. This manifests in a few ways:
1. Always using the brakes results in hand fatigue. In my usage I only have couple long stretches where I'm on a constant slope, but I'm on one of the brakes at least 95% of the time. I'll state for the record I've broken one of the brake levers, I guess just from use. I happened to be at a flat section, so it wasn't a problem, but again given your catastrophic consequences it means you'd probably keep a spare on hand. Anyway, I at least get to alternate which brake I'm using, but since it's all on the right hand, it does wear you out over time. I've pondered stripping down a motorcycle braking system to convert to hydraulic, but it's gone no further than pondering. Maybe this winter I will have some time. I've also thought of taking my one spare brake lever to a shop to see if I can find a nice stainless replacement and not have to worry about braking a plastic handle.
2. Even with the brakes, the machine doesn't track exactly straight, but instead you have to point it up the hill a few degrees. In theory using the diff lock could alleviate this (and using the brakes!) but engagement isn't 100% on the spot. I've found this out the hard way getting stuck, and not having the lock work. There's probably a trick to get it to engage straight away, but I don't know it. Because my sloped sections are relatively short, I've never bothered to try it with the lock on. But I will next time I mow to see if it gets rid of the skewing problem. Depending on how slippery, bumpy, soft, etc the land is, eventually it's going to want to point downhill and you'll have to point it back the right way.
3. Overall machine quality is high but small parts tend to break. I think the diesel vibrations (not to mention the cutter bars) are hard on most parts. I've lost a variety of bolts, nuts, seen things loosen up, broken, etc. The only two potentially critical failures (which weren't for me, but I wasn't on a 30* slope with a time limit to get the job done) have been the brake level, and the handlebar height adjustment pin. Once that breaks or walks itself off, you lose up/down leverage on the machine and gravity has its way. I've looked at a superior pin that's both longer and with c-clips (instead of press-on clips) but haven't ordered anything yet.
Main point is, have spares on hand in advance for anything that you really can't do the job without. Stuff will break, and usually at the least convenient time.
All things said, these were made for farming the hills of Europe so if there's any solution to do what you need, I'm sure a 2-wheel can do it. Good luck!