For the home-built special, why not consider the use of hand throttles similar to that used in a twin engined aircraft? if established with a electronic throttle control, you would effectively have easy control of both engines at the touch of your fingers.
I've considered something similar for varying the degree of engine braking in an ICE. By varying the degree of EGR, fan braking, air-con temperature, you can potential vary the degree of engine braking experienced. [CAT haul trucks have their braking system lever next to the steering wheel. the emergency brakes on the older models are sprung loaded- perfect for braking on the straight.... and some corners]
If you were to maintain the pedal accelerator for the ICE, and still incorporate a hand throttle for the electric motor with forward and rear movement (neutral point- electric motor off, rearward-> increasing regen braking). A fail safe, similar to that used withcruise control would be needed to automatically stop forward electric motor propulsion when you touch the brakes or clutch for example.
The tech people would need to clarify for me how you would wire it up so vary the regen braking available.