Here’s a random idea:
PMs could be a category. Just one. A special, default one, part of the package like Lounge and Meta.
It could use the same principles as the approach suggested for topic archives:
So when you PM someone, your topic ends up in this special “PM Category” that isn’t visible in the normal drop-downs, not even to Admins. So besides the fact that it’s essentially an unlisted topic shared with whichever participants have been invited, you can tag it, wikify it etc., just like you could with any other topic.
(sorry if this seems off-topic, but I think it’s very much on-topic, since we’re discussing whether OP’s suggestion is better off dismissed in favor of PMs, which I’m arguing is not the right way to go).