It's pretty trivial to get postfix (and I'm sure sendmail) to send all users @ somedomain that don't match "real" users to a "collection" user. Then have procmail run on that user's mail only.
It's not something you can do without root, but the mta part is so simple to set up I'd be surprised if you couldn't find a host willing to do that for you. The procmail part of course wouldn't require root.
