I can't think of any forums where I have seen the opener able to open and close the thread and the same being unable to reopen a thread the mod closed that they started. For your mentioned purposes, It you can post "game full" and make another post latter saying a spot opened. that said: the starter of the thread being able to close his own thread (if that can't be done right now, no experience starting a thread on roll20 personally) would probably be a good idea, even if they couldn't open it again.