Well I'm confused. You're only concerned with post
titles? You think Google finds post titles mentioning PR link trades and punishes sites requesting link trades based on PR? What if webmasters made their link trade requests via e-mail and not in a post title?
Unnatural linking patterns? What's unnatural about a PR4 site having a link trade with a PR4 site? I haven't studied anything at all about PR but I don't see why Google would punish a site just because it requested a link trade with a site with the same PR as it's own. Since no one knows what the actual algorythm is, we can only guess. I think Google is more concerned with relevancy to search term/s.
From what I've heard PR is overrated. My free sites get a LOT of traffic from link list pages that have PR0 and PR1. That's enough for me not to worry about PR.
I don't know anything about all this so my questions are just to get information.