Nice one Kitnut - The thing that's always struck me with multi-wheel drive designs is that hydraulic motors would be the way to go, i.e. the engine drive a hydraulic pump instead of a gearbox and that supplied pressure to motors in each hub. I know there are issues to solve about heat, but it has been done a number of times, so the issues must be solvable.
I built a bike for a commission at the start of last year, and a couple of things struck me:
1. The bike model aftermarket parts scene is almost non-existent. You can get stuff like cables and pipes that mainly intended for cars, and there are few specific upgrade kits for a few models, but it's nothing liek the aircraft or military vehicle scenes.
2. It's remarkably hard to customise a bike model, partly because the lack of alternative parts (see above) means that you have to butcher another kit to get just about anything, and partly because of the way they're made. When you're "chopping" a real bike, one of the easier jobs is making a longer chain to deal with the fact that you've either increased the engine-to-wheel distance or changed the sprocket sizes. On bike models though, this is very hard, because the chain and sprockets are usually moulded in one piece, and even if you grafted extra bits of chain into the middle, the angle that the chain comes off the sprockets would still be wrong. There are aftermarket "real chain" kits, but they cost the earth and are geared towards specific racing bikes, which means they're usually quite short.