Make sub-forum post/thread counts add to parent forum on forumhome?

Does anyone know if there's a hack (or a way to...) make a sub-forum's thread/post count be added to the thread/post count of its parent forum on the forum homepage?

For example. If I have a forum called Parent that has 50 posts in it. Then there's a sub-forum called Child that has 100 posts in it. I want the forum homepage to display 150 total posts in that forum instead of just 50.

I don't think templates can handle calculations... so I rekon you'd need to be ammending some php files somewhere.

Nopid is here somewhere and is fast deserving the accolade "vBW PHP Guru..." - I am sure he will drop some info in here when he reads it...