<joke amount = "half">
Between, Super Search, Levels of Monks, and by following the dates, it can be approximated. (Voting/Experience System explains how points are distributed.) Of course, you'll need to vote on each node, which might take a while and would have to be incorporated into the approximation. XML might make it a bit easier.
Or, you you could use The Wayback Machine like eyepopslikeamosquito did, and check the users' pages, if available.
For the older monks, don't forget to incorporate the old leveling system.
I'm kind of interested in this. Specifically, to know what level a monk was at the time of a specific post. So many forums tell you what the user is now, which is redundant, misleading, and not applicable to the post being looked at...unless it's to say, "hey, look at where a post like this might lead." And, it has an extra cost on the system to keep it dynamic. So silly. :)