Not a bad idea. Lossy compared to the 5th-wheel setup, but compared to low load operation it's a win-win. IIRC, I think I fiddle around w/ the numbers and figured that w/ lead acid batteries it would be worthwhile w/ gas at $3/gallon even if it only went up to ~10-15mph, just because BSFC at those speeds is crazy high.
GM's BAS supposedly increases the efficiency of the Vue by ~20%. If you could fit one, a modded ~3-4hp 36V golf cart motor would be great. You could toss in a ~$600 LiFePO4 pack if everything works out and have something that'd be a half to a quarter of the cost of gasoline to run at low load.