Probably the best place to keep this info is the Halforums Wiki:
Halforums Wiki Reference
For instance, if you had no idea who wasabi poptart was, you could merely use the little searching box in the upper right of the wiki to find out. Alternately, if you wondered where the heck wildsoul went, you could use the search box and find out that she changed her username.
If you haven't the slightest who I was, you could search of First Lady Puffinstuff and find out.
It does require people to maintain it, but given the new cost of username changes, I don't think they'll happen so often that we can't keep the wiki up to date. Would be nice if a mod could occasionally check username changes and update the wiki for us, since the mods have nothing to do around here, given we are such a congenial group of people. ;-P
-Adam