For what it's worth, I went ahead and did the manual work myself. It took a while, but I managed to work out a collection of scripts that convert an ancient install to a state where it can be upgraded by UBBThreads. I managed to get most stuff across, users, boards, posts etc. but some stuff did go missing (file attachments), and permissions needed to be re-done for each board afterwards.
If you're ever in the same situation, drop me a line (PM or email at mark underscore round at ipcmedia dot com) and I'll let you know the details (the scripts are largely specific to our situation, but I'll share what I can).
-Mark
