Well, I'll admit that my statement was a bit over-inclusive of surge brake systems regarding the lockout "key." (Kindof ironic since I'm always getting on others about "over generalization" - oh well - guess I'm allowed to stub my toe once in a while too...)
In actuality, all surge breaks require some type of "lockout" - the "key" system is just one method (cheapest/simplest). There are others, but they add varying degrees of complexity/cost. Different manufacturers approach the issue differently, hence there are several methods of addressing the backing problem.
Most trailers I've seen simply use the lockout "key" which you physically put in place when you are ready to back up. When you start going forward, the key falls out (it's tied to a cord, so it doesn't go far) and then you have brakes again. As far as your situation, yes, there are a couple of other ways to address this issue.
I've heard of (but haven't seen) of a system where the rear shoe is hinged - when backing, the reverse rotation pushes the shoe away from the drum, releasing pressure (I guess through some type of cam setup.) DICO might have been the company, but don't quote me on that. Sounds like you have probably used something similr to this in your experiences.
Another way to address the issue is an electrically based solenoid solution that uses the reverse light signal to actuate a solenoid/valve that prevents the brakes from engaging while in reverse (e.g. "automated" lockout - same function as a key, just no getting out to operate). Of course now we are getting in to "intelligence" (e.g. electrical signals based upon the driver's wishes), so we are starting to get away from a completely stand-alone surge brake setup.
I have also seen at least one other system advertised that is supposed to add quite a bit more intelligence. It retains the hydraulics of the surge brake setup, but removes the "surge" from the system. Basically it mates an electronic controller (as with traditional electric brakes) with the hydraulic actuation of the surge brakes. To me, this seems like the "best of both worlds" as you don't have to worry about magnets & such "going bad" but you still retain all the intelligence/adjustment associated with the electric brake controller. Course, this could be a dud (haven't seen it in action), but seems like a good idea. Now we're way beyond surge brakes setup, as there is no "surge" left in the system.
Most surge brakes have no braking ability or a fixed braking force (i.e. I've seen 20% advertised) when in "reverse." Again, for flatland ops that wouldn't be THAT big of a deal, but if I have to stop half way up a mountain pass, I want all the braking power I can muster - true surge brakes won't give it to me, or give me some limited "fixed" amount (e.g. 20%). Not good enough for me when I have a heavy load. (Don't get me wrong, I don't make a habit of stopping on the side of a mountain, but it has happened due to necessity in the past.)
Again, I have both, and believe it or not I do like my surge brakes - but only for very specific circumstances. For general utility/towing use, I believe that electrics are the way to go. They're simple to install, give the operator a far greater degree of flexibility to adjust to the load being carried, and work equally well in both in forward and reverse.