I'd have to second this request for this hack. In fact I can't believe nobody has addressed this before. If I had a clue about php I'd do it myself.
I have the exact same problem in that I have to move all threads to a separate forum that users cannot see. The key issue here is SPACE. The thousands of posts that have accumulated there occupy more space then all my other forums combined! I want to be able to flush out that forum (ie HARD delete all those posts), but NOT impact on users post counts.
Once a post has been made, the act of posting has been acomplished, thus +1 to the post count. The physical removal of the 'evidence' of the posting should not have anything to do with the post count.
What amazes me is some feel so strongly that post counts should be -1 when a post is hard deleted, yet they feel that soft-delete is fine. It's no different from the users point of view, he has still committed the 'act of posting' weather his post is later hard deleted OR soft deleted, so why should his post count be effected be either?
I'd really appreciate it if someone would take a look at a hack that could acomplish this. I would really appreciate it. It will put this descision in the hands of the admin, where it belongs.