can save some typing and also the effort to hack email notification part, because after db insert, his userid is sent to sendnotification function and thread subscribers learn his real identity if we dont clear it.
@Blak n Wite: BTW I wonder why you dont just enable guest posting in that forum? So if someone wants to post without his username, he can just log off and post with a guest account.
@filburt1: nice idea but does not work: an insistent user can still track the post user with Search function (show posts), user profile (which shows the last post of user), also search all posts by user option. So database modification as Xenon suggested is necessary and this makes hard to learn his identity for admins too. Your one chance may be enabling email notification for mods in this forum and send his real username with new post notification mail. Other solution would require db alteration which is not recommended for this small hack..
I don't want to enable guests for that forum, because guests can't see the forums Only registered members can