To be honest if my flatmate sent me a text like this I'd probably say yes, "provided I can't tell he's ever been there" or something like that. But I can't imagine that when they asked me the question they would downplay the importance of it. I can imagine them saying something like "I know this is really cheeky of me, and feel free to say no, but..." It seems unfair if you feel bullied or backed into a corner about it.
Maybe you could lie, saying something like you might well be back in the next few weeks for some reason or other, like you might try to get a job there, or need the room while he's there, anything to make them think him going in somebody else's room is a better idea. If everyone else's room is occupied then I agree that the best thing for flat politics would be to say yes...
I don't know the background but it does sound like you're being a bit oversensitive about this... Have your flatmates been mean to you in the past? And you haven't met this guy, you could always ask for his number and send a friendly text saying can he make sure he keeps it tidy or something.