[Dr. Jerryrigger]

I've thought about this (but haven't done anything) and came up with a few possibility:
1- mount it where the AC compressor used to be, easier for me than you, as my compressor is in a box in the spare bedroom. There are a lot of complications with this, and you seem to have spotted most of them. And I don't really know if that little belt is idea for transferring more than 5hp

2- mill gear teeth into your brake disks (2 or 4 of them) and mount a little brushless motor to you strut mount. This would require a lot of machining and you would only be able to gear it for the highway, but that might be okay...

3- (the one I'm considering at the moment, if only I had the $$) A electric assist boat tail trailer. Two birds with one stone. It would be a little hairy having a trailer pushing your car, but with the right linkage it could work out. This idea needs much more planning.

Edit: Speaking of boat-tails, can you hit me up with a link to your boat tail thread? I found it via google some time back, but I just skimmed it then. never mind, found it.

For manual controlling of the motor; I was thinking a T-handle throttle from a boat would do the trick (and with style), with a switch to put it into a charging circuit for slowing.