I think that you'd just make a three position switch normal, off, regen. Where normal: the voltage from the ingnition switch goes to field like normal, off: the voltage does not go to the field, regen: the brake light relay goes to the field.
I think the field is there so: the engine turns off when the ignition turns off and so the field voltage does not drain the battery when parked. Otherwise it would just be hooked to the battery full time.
I don't think you would get much stopping power as a 60 amp alternator is only a little over 1 horsepower
"Any thoughts on how to accomplish this or what sized motor would be needed? It'd be great to just engage a big 'ol spring between the rear wheels when slowing down and then releasing its energy as needed, but I don't think that'd work all that well."
I think an air compressor and tank would be lighter than electric motor and battery