There are a lot of ideas here about technical ways to make it easier- but what about relying on the mods/users to see and report (and in the mod's case, deal with) any violators of the "no spam" "no link spam" ilk? It's usually pretty obvious if someone is trying to link spam (and general spammers are a no-brainer) and in most cases, will get caught pretty quickly and can be removed/banned from the site. Sure, the mods can't catch everything, and shouldn't be expected to catch everything. But most regular readers of WebmasterWorld are pretty hardcore from what I can tell, and I doubt much would get by unnoticed, if at all. The technical ideas of what/where/how to allow links are good, but really may be overkill in this case.
Of course, you first need to decide in what manner/types of discussions links will be allowed, and in my mind, that's actually the hardest part.