It's very easy to do to be honest. However it's very easy to annoy users and run them off too. Many users don't like seeing globals, especially if they can't block them. Think about how annoying it is to have someone in your channel that has a repeating away message, like someone really cares you're away, let alone cares to be reminded of it. For me personally, that's the #1 way to get kick/banned from my channel.
Perhaps it would be less intrusive as a timed WALLOPS message, that way users can at least set umode -w.