I think what you'd want to do is something like add a column to the user table and use it to save the threadid of the reputation thread when it's created, then use that value to create a link in the postbit. Or you might be able to use a profile field instead. In either case you could write a short piece of code that would run once to set the field for existing users by doing queries on the thread table as you suggested.
Yep, it should be coded in a method similar to yours @kh99. I was just stating the process of obtaining the data as an example. In reality I would have coded something similar, eliminating the extra query. Although on my board, adding to the user table can be more of a challenge.