I dunno about a shunt wound motor, but I think PM motors are too wide to package inline with a RWD output shaft at the power outputs needed, although they can do regen.
IMO the easiest way to do everything is to drop a small diesel into your S10, run manual accessories, put a series DC motor behind the trans, and run regen
off of the driveshaft (Unfortunately the original link is gone but all the basic info is there).
You would need to fabricate mounts and an adapter for the engine, DC motor, and rewound alternator, plus all the stuff on the EV side that you're already familiar with. It would be the least amount of work, but also less efficient and slower because the DC motor will be pulling more current at lower speeds than one connected to a transmission, and the engine would need to go through the relatively inefficient RWD setup to put power to the road.
Running two separate FWD setups would be more efficient (unless you choice of transaxles is really limited), but you would need to fabricate way more stuff, and unless you could automate the shifting of the motor somehow (without an automatic transmission since that would trash any efficiency improvement) that would be a pain, although doable since the engine and motor would have significantly different shift points. For a common and suitable pair of VW transaxles (ASF for the engine and GC for the motor) the shift points for the engine would be at 10, 20, 30, 40, and 50 mph, while the transmission for the motor should shift at 25, 40, and 60 mph for best efficiency. It would be kinda weird if you were using both, but I guess e-2nd, e-3rd/m-2nd, e-4th, m-3rd, e-5th, and m-4th wouldn't be impossible. Kinda like driving a 4WD vehicle or big rig, just a bit more complex.
Oh, and spinning the output shaft of a RWD trans shouldn't induce too many losses, at least not enough to consider putting another clutch inline IMO.