So i have a bunch of thoughts on this!
As far as how you easily shorten a manual link, you pick up your implement and then set the rear of it down on something above ground level. Random rocks, tree stump, curb, etc and i would guess many implements have room onboard for a brick or 4x4 piece of wood etc to make this happen anywhere.
As far as top link design ideas:
1. 'Telescoping' top link. The pieces below are actually ride height 'mockup' tools intended to go in place of coilover shocks on a car to 'mock up' different ride heights for testing. They are about the same size as a cat0 top link, and built lighter than a 'normal' cat0 top link at that. But i do have one in place as the top link on my little Case 444 and will see how it holds up. But i think this idea for a larger top link would be good for large, fast adjustments. It is still threaded with jam nuts for adjustment in the usual way.
2. 12v 'Electrohydraulic actuator' as top link. A hydraulic cylinder with an onboard pump and reservoir. These are hard to find and expensive, but SUPER convenient as a power top link IF you can find something suitable. I have one installed as the power top link on my little Kubota B6100. This pic shows the first installation with the motor on the top outboard end and only one spherical joint at the tractor end. I've since welded another spherical joint to the base end and flipped it so the base end is attached to tractor. If these were more widely available in more sizes they'd be the go-to for people with no remotes, but the one im using would be far too weak for anything on the larger end of Cat1, let alone Cat2. I doubt it could tilt up the end of my bush hog, for example (haven't tried) but it doesn't really matter because i can always set the back of the implement on something as i said above (even the pile of dirt in the pic would work) and let gravity assist the cylinder that way.
3. A screw jack such as an adjustable side link 'leveling box', but as a top link. Or, basically a trailer tongue jack with spherical rod ends as a top link.
4. Ratcheting chain/load binder modified into top link. You can get a pretty big one of these for <$50, and you've already got your normal top link, right? Well, cut the middle out of this and weld it into the middle of your top link, now you have a reversible ratcheting handle on your top link. They make folding handles as well. I bought a small one to do this for a cat0 top link but i ended up converting it to a 3pt stabilizer/tensioner on my b6100 instead. Could also work as an adjustable side link in the same cut/weld fashion. So there's 3 uses right there..
5. 'Hydraulink'. Now this would be extra fiddly but there is no magic i can tell to recreating the 'hydraulink' out of a hydraulic cylinder. The only reason you can't just hook both ports of a hydraulic cylinder together with a needle valve in the middle is because they typically have different displacements on each side of the piston due to the presence of the cylinder rod on only one side. Well, there are two ways to get around this. One is leave some air in the cylinder. The cylinder will still be harder to move one way than the other, but like i said up top when you've got a heavy implement, gravity, and leverage involved it will still work by just compressing the air to allow fluid into that side. The fiddly bit is figuring out how MUCH air to make it work 'right'.
But I don't think that's what actual HydrauLink pictured above does, although they include some air/gas for cushioning. The other way to make it easy to hook both ends of a cylinder together through a needle valve, is just to use a 'through rod' cylinder with rod on both ends, which makes both sides have the same displacement. In that case you don't need to leave any air in there and it will just work.
So if you notice the hydraulink is actually a very short ~6-8" cylinder with an extension welded on one end. I suspect internally it is a through-rod cylinder with one end possibly cut/shortened to not interfere with the threaded rod end screwed into that end of it. It may also include some intentional 'gas pockets' for cushioning.
Which, by the way, if you leave air in a cylinder it has to be on both sides of the piston or it will only cushion in one direction. If you hook both ends together, how do you keep the air on one side or the other, or both? Well, suspension shock absorbers which are another form of hydraulic cylinder trap their 'gas' either in separate chambers (outer housing of a twin tube shock, gas piston at base end of a monotube shock, or even an external reservoir) or in 'gas bags' or gas bags in separate chambers, but one possible way of doing it is simply to trap air in closed cell foam. Some shock absorbers do this:
video
You could make such closed cell foam take the shape of a 'bumper' aka bumpstop. One 'donut' shaped bumper of closed cell foam on each side of the cylinder would trap air and cushion the cylinder movement in each direction while not allowing the gas to travel from one side of the piston to the other through the valve. Some shocks/struts do include a bumper like this internally for their extension travel aka 'topping out' (vs bottoming out).
I don't even know why the cushioning is important but Hydraulink says it reduces operator fatigue, so heck if you want it just put it in there i guess! You could also plumb in actual hydraulic accumulators but that would be a lot more expense than some air pockets/gas bags/foam.
