[ecomodded]

Its a good idea.

If it works it works,hillbilly or not. A lot of the greatest engineering problems are solved by the most simplistic method imaginable. So simple is good.
Especially good for those with little to no funds. Might be able to make it from 95% recycled parts. Which makes it even greater of a discovery.
If you think you could weld a pulley to the drive shaft,keep the welds in balance, also check with a welder. Off hand it must throw the balance off by a large amount, unless some how the torque keeps it in balance.. i do not know!
I think your idea of going off the transmission output sounds really good, could even weld it off the front u-joint's backside, nearest the tranny.
Regenerative engine/transmission braking. You are onto something.

Most cars travel at about 2500 to 3000 at 70 mph.
In top gear if your motor's RPM are 3500 then your driveshaft would be about 3500, if your high gear is 1to 1, which i think it is,but you should check.