I agree totally with miniflyer. not impossible, but very problematic. in full scale engines, theoretically the core or spool should operate independently of the afterburner... that is to say core speed and temp is only marginally effected when throttle is advanced beyond military power in to the ab range. in a turbojet, there are systems to deal with (theoretically) ab backpressure to avoid compressor stag conditions....though they do happen sometimes. but more often you have( in turbofans) ab pressure spikes that can and often does backtravel the bypass duct and stall the fan.

Back to our model motors u, have the additional requirement of a flameholder to increase local turbulence and allow for more burning before exiting nozzle. now a nozzle for effect with no ab function, is far less complicated

In fact, nozzles do operate even when not in the reheat state... so if u found a way to mechanically build an nozzle... the nozzle off state would be wide open.. nozzle on state would be closed....for a few seconds at a time just for effect then heat probably wouldnt be an issue...but again this would not be an afterburner. just a variable nozzle. but i would probably expect points on scale look and not bet on increased power. would be majorly cool though!!!!!!!!!!!