Give me... 3 days.
It looks to be a moderately easy fit, either new bearings and spacers, or a great big captive spacer going through the whole thing, sprocket carrier and all. I think you need to grind down the brake caliper hanger too.Plus, you need to either fit a 530 pitch chain and front sprocket to match the B12, or find a 525 or 520 pitch sprocket for the wheel. Seems that GS500Es might have a compatible 520 pitch one. Supersprox don't make one to suit yet though it's on their list of possible new models now. Renthal will though.
B12 wheel weighs as much as a planet though...