Theymos, have you considered upgrading to 2.0? It was released over a month ago. I don't know if the they support such a feature in the new one but they have a lot of nice improvements added and over 2 years in RC's (even longer in betas) it seems pretty exciting for them to have a final 2.0 out.
It would probably be a big project. I'd want to keep the same forum style that we have now, and several of the modifications and custom changes that have been added would break.
Can there at least be put a message shown when sending a pm, you have N pms left for this hour?
I would if someone would contribute that code. Though if you're going to contribute code, you might as well implement membergroup-based limits.