well said - excellent post
For just goofs and giggles, if someone wanted to put a D or E on the back end of a plane, simply for a straight-line burst of speed, you would have no real need to fold the prop - the engine would be happy to unload for a few seconds (note, maybe 2-3 seconds of engine burn is considered a long time for most high-powered model rockets). Nothing new here, it has been done in the past. Done properly, you may see a momentary speed gain of 20-30 mph or so. I guess this could be fun in a speed trap.
Going to a full hybrid rocket motor on a purpose built aircraft design --- that is a different story. In which case - if the goal is to fly the plane to a starting altitude using more conventional power, then the above mentioned idea of electric with a folding prop (ducted fan might work ok too) is probably the way to go for both function, in-flight-restart, and drag reasons.