Dumb question
 

Would it be possible to mount an electric motor, through a sprag clutch, to the accessory pulley on the engine (in parallel with the engine)? Select and optimize the electric motor up to run at your average cruise control speed (55mph), providing perhaps 2-5hP. Engage the electric motor only with the cruise control. That would allow you to statically set the current and voltage, since meaning the motor will not try to track with load (the engine, on cruise control, will do that). The idea would be that you could get a 48V, 4 battery system to provide supplemental power for 1 hour per charge. If an electric motor were to provide 3hP of the 15hP you needed to cruise at 55mph, you could perhaps save 15% of your fuel consumption at that speed (yes, I know that 3/15 = 20%).

What do you think? Am I missing some key concepts?

Yes. It's been done, on a Ford explorer. Don't ask me to find where I found this, it was on some forum somewhere on the internet. Start searching.

That is kind of how IMA works on the insight. . .but it does it the other way around. It holds the gas engine at constant load output and then if it needs to accelerate bumps the electric on to pull to desired speed and then switches back to ICE only.

Its not static, but its more or less pulling as hard as it can when its on.

Similar to the Lotus developed stop/start system where the starter/alternator motor is connected through the accessory belt with the idea being easy retrofit to many makes of vehicle.

you can calculate
 

In the fossil free forum sticky,I posted enough data such that you could calculate the energy balance for your proposed mods to see if it makes sense engineering wise.

Thanks! I'll check it out.