I think people who exchange links do this for PR mainly. And what the reason to exchange links with a site that has no PR? May be it is new but may be it was banned by Google.
I start to exchange links for a site only when it already got PR. The site got PR from open directories (DMOZ, JoeAnt and others) and my other sites.
May be it's not good to prohibit to exchange links for new sites. May be the forum should be splited for new and old sites. Than it will be easy to find required sites.