The regen function is combined with the power cutoff function which is activated by both the front and rear brakes.
The power cutoff function is a legal requirement on ebikes in many countries, so preventing it from working via the brake levers would probably make your bike illegal to use on public roads.
The green horn button could be rewired in parallel with the brake lever switches to allow the regen to work by simply holding the horn button depressed, but this would have to work in addition to the normal brake lever activation to keep your bike legal.
If you
don't have a pedelec sensor fitted, I think it might be possible
(and quite involved) to reconfigure the brake switch wiring to the throttle signal wire and cruise signal wire in conjunction with a 330 ohm resistor and a couple of diodes so that the brake switches simply cut the throttle signal going to the controller
(and also disengage the cruise function if engaged) instead of activating the regen. This should still conform with the legal requirements of cutting the power with either brake lever.
The horn button could then be reconfigured to instantly operate the regen when pressed.
Alan EDIT: Theoretical circuit diagram added
