It might be possible, however I don't think it's really feasible with the current system. What I mean is, say you have 25,000 users. If you send out a PM to all of them, the DB would need to insert 25,000 rows into 3 different tables. That's a lot of database activity, and on some systems might even go into a timeout.
I think a better alternative would be to do something like we do with the new forum rules, where the user must accept them the next time they come online.
What we could probably do, is add some sort of system message function. Where you create one or more system messages that the user must view the next time they login.
