Usergroup dependant on posting in certain forum

[Caiman]

I am going to be installing Logican's fantastic announcement hack shortly and it got me thnking about users who havem't posted an introduction in our 'into area'- I'd like them to see a reminder annoucement for a set amount of time or until they post a new thread in that section

Is there a way I can place all new users in a 'haven't posted intro thread' usergroup which they are automatically promoted from into the regular 'registered users' usergroup once they have? (posted a new thread in a certain forum)