I was thinking about running a group game but I am still gathering ideas. I will just post some thoughts (about the 'technical' things, not the plot) and maybe one of you guys who has already done it can chip in a little.
To face the player On/Offs problem I want to make an 'official' list of kinks that anyone who plays has to consent to. So, for example, D/s (or anal, gags, whatever) could be announced to be 'canon' in the roleplay, so it is not necessary for everyone to check with their partners first. If it is on the list, you are free to go. Exceptions could be allowed (people who don't like everything on the list), but they would have to specifically mention it. Conversely, a player intending to do anything outside the list would have to check in with their prospective 'victim' first.
In my game, I was thinking that sexual dominance and submisison would depend on rank and personal success during play. Therefore, as success can not be guaranteed, all players would have to be okay with the possibility of being on the bottom. It is possible to play for failure, though, so submissives might be acceptable but pure doms would be not. In the same way, by public announcement, I hope to deal with the player gender/character gender problem. Everyone will know that they will have to be fine with mixing it up, or they shouldn't come in the first place.
I like what AndyZ said about entries&exits. It is always a good idea to work with what is going to happen anyway instead of trying to avoid it at all costs. I'm not sure if this is applicable to a smaller group game, though.
To find reliable players, my idea was to do a kind of introduction. A 'boot camp', 'mission 0' or whatever of limited length. People coming and going in this phase would be no big deal. And only those who play reasonably well and reliable would be invited to join the game proper.
I'm unsure about the system. Freeform works well enough for me, but I also like the sense of accomplishment and the randomness a system game brings. Players can try and see if they succeed in a system game and be rewarded for success in a more palpable way than in a freeform. On the other hand, I really don't like the bookkeeping and delay a full blown system (even a relatively simple one like D&D or WoD) brings to a game once the combat starts. So I was thinking to use a very, very simple system that can deal with a small combat in just 2 posts or so while still having a random outcome that does not seem arbitrary or unfair to the players.