Nope not a "Rube Goldberg" setup as you say, it is the only way to reverse the rotation so you can power it from the left side of the bike with the engine output on the right and the engine running in the wrong direction. The other two ways is to turn the engine around and have it as not pleasing to look at or build a two gear gearbox to reverse the motor rotation before going through a jack shaft.I thought dracothered was suggesting the power flow was from the engine pulley-jackshaft-drive pulley on the other side of the seatpost-sheave or chain to the rear wheel. With the engine output shaft on the right side, this would make the bike go backwards.
If the plan is to drive through an idler pulley off the back of the belt it would work, but then you'd have a pulley spinning for no other purpose than to route the belt back to the engine....seems like a "Rube Goldberg" set-up to me.