Should there be a valid reason for being anonymous for a post, all you have to do is "delete cookies," or just any cookies that have mudcat as part of the name. To return to being a real person, just log in again. You can copy (or export) your mudcat cookie(s) somewhere, and then just paste them back (or better- import them back) when you want to be real, but you should keep a recent copy handy if you do that.
Isn't this thread something like the "Guest register" some sites use? Maybe "Guests and Inactive Members Hello Register" - but we'd need a note in the FAQ that Inactives can note occasionally that they're still around. (And a way to get people to read the %$!@))! FAQ.)