First - thanks for starting some very inspiring threads Ben!
DIY and low cost equals to DC these days - it means no regenerative use of motor... So I'd rather consider hybrid-plugin setup, like your DB diesel for high speed cruising and electric for all "unstable dynamics" like acceleration or stop & go traffic.
This way you'd start from you driveway with electric motor, then pass trough a few crossings and STOP signs and finally you get to 4 - 14 - 114 miles freeway part of route. As a conscious user you decide then to switch to diesel for this part because it's what diesel likes - long stretches of constant conditions... (of course it was already warm enough thanks to mains-fed electric heater with programmed timer...)
Yo're coming to destination town after consuming a few liters of biodiesel, being a conscious driver you flick the switch 700m before freeway exit so next acceleration is pure electric and so on to destination point...
This means there should be choice between drive source "in the fly" so some clutching/coupling system needs to be designed, knowing characteristics of available ICEs and electric motors I'd place this clutching/coupling device in front of transmission. But how? First idea is to have chain sprocket on transmission input shaft, second sprocket on separate driveshaft parallel to main axis and pointing rearwards, then some kind of "teeth clutch" (sorry, forgot english term for it) connecting to DC motor under rear bed.
We as ecomodders and DIYers have an advantage compared to GM/Toyota/other hybrid mfrs average customers - we can do with our brains what they don't know or don't care, relying on car's brain to do. It means that with few relative straight mods or builds we can achieve goals that manufacturers have been chasing for a long time being tied with average mental and knowledge levels of their customers. At least at designing and reserach/test stage we can steer our hybrid drivetrains manual way
