I’d love to do a ‘back to back’ road trip with a Golf TDI or something like that and compare highway mileage in the exact same conditions.
Thing is, the hold mode added for the Volt in 2013 was a tremendous improvement, and I always use it on the long trips to preserve the battery for the slower speed driving at either end of the trip. Even so, the car is collecting regen energy on hills or during deceleration for slower portions, then allowing the battery to kick back in, without depleting the remaining charge, for going up hills or passing. This way the gas engine can just lope along most of the time, much like a big V8 would do against a tall gear.
The only drawback I can see with using it in a truck application is the packaging of the battery. Not being an automotive engineer, I can’t really envision where they would put it, particularly if you had all wheel drive. SUV’s on the other hand might be a more viable place to try it out.