Hmm, I think that wouldnt be too hard actually- stick a 1/8 diff in the tranny, but have some special 'coupler' attached to the ring gear (doubled stacked essentially). Then, use the gear shifter mechanism to engage and disengage something like the dog shifter that is used in the stock tranny- only its fixed to the rear output shaft- thus locking the rear shaft to the diff cup and making it act as a spool. Unlocking/ disengaging it makes the center diff act like a center diff again, with the front wheels able to unload some power when you mash the throttle.*
*my design, buy the rights off me for $1000.