Today, I stumbled across this drawing of the "Via E-Rev" powertrain.
Looks like it's what the Volt was supposed to be, only in a pickup truck.
The funny thing that I noticed is that it's exactly the same physical configuration of components that I was thinking of for a DIY Hybrid truck. The only difference is that the engine goes to a generator, and then to the motor, instead of through a transmission and physical connection to the motor. (So it's a series instead of parallel, even through all the components are in the same places and orientations.)
The downside to that is both the engine/generator and the motor system all have to be working at the same time. In my proposed setup, I would be able to drive on the diesel system while I'm working on the electric system.
Note that the drawing does NOT show where the exhaust system and fuel tank are. On a typical pickup, both of those are where they show batteries.