Rib the ABS tube like a vacuum cleaner hose or plastic conduit. No need to be tough so long as it can get out of the way. A ribbed tube can be thinner and therefore lighter too, plus the ribs trap grease and add an additional barrier.

What you can't do is add it as an afterthought. The MZ one works because of the flanges on the back of the motor and the ribbed/bellows section at either end. Nothing gets in, the grease in there is as fresh as the stuff in the rest of the engines enclosed spaces. The Harley version and other add ons fail because they try and use gravity to hold the case up to a motor.

I'd need to look in detail, but I'd also bet you could make a lubed chain smaller. There will be a safety factor that assumes an open chain will be rusty and worn out. If you are sure the chain won't get to this state and will be enclosed if it does fail in steel wire running round the case you have two choices. Either use an industrial chain the same size (cheaper) or use a smaller motorcycle chain. The weight can come off if it's that vital at the design stage.

Shaft drive is fine on Moto Guzzi's, Urals and Honda's where they control the lubrication and think about how it will wear. The BMW spline thing is garbage, even if they fitted grease nipples.

Andy