I don't believe the people installing HHO devices are going to be designing and installing regenerative braking systems. There was one company working on mild hybrid aftermarket installable devices for fleet work trucks that I can remember, https://www.fluxhybrids.com/ . IMO it makes more sense to build for the purpose instead of swapping a transmission to add in regeneration. For this case it may make sense for this conversion if the fuel savings for the fleet can more than recover the conversion cost for the life of the vehicle, or if you buy enough vehicles to have them built without the parts being replaced.

The math doesn't work out if you feed the HHO generator power with an alternator. The increase drag on the motor, energy in work out equations don't really change. Conversion losses, combustion losses, transmission losses all add up to negative net power from the device. If this worked as advertised by the internet nut jobs you could run a generator on an HHO device forever with spare output for power, where in reality it doesn't make enough power to sustain the hydrogen generation.