There is no problem with this: you can prevent this domains to be used (you need to code a few lines in php) and you can have a text file or a table indatabase with the subdomains you want to prevent for being used by users (ftp, mail, dont worries about that, belibe me I did it before and is easy)

My really fear is any problem with sessions bettwen subdomains. I need to prove, I will try to do it in the next days and I will come to you with more info!,