[JohnHorton]

It seems like one could build it without any "brains" at all. In fact, I think a KISS design with, say, a light switch turned on by the hostbot and turned off when impacting the target, rigged to a motor directly might be the best idea. They were very specific about having " exactly one 12V rechargeable NiMH battery pack...", so it would seem they want this to be electrically driven as opposed to something mechanically launched from the hostbot.
Unless you just stick a battery on your projectile minibot. That'd be legal.