I assume no one's done anything with this since the topic was started? Certainly my searching didn't bring anything up.
I'd also like such a feature. I used to run a BBS back, back in the day, and we had several boards where people could post anonymously. Of course admins could see the source, but other users could not.
It's a great feature that allows people to say things without so much fear of repercussion (in my case, we had a sex talk forum and people are way more likely to be honest and open when they're not being judged). No modern software seems to allow this functionality.
I can't code worth a damn, but I'll cheerfully offer twenty bucks if anyone wants to get this done. =)