Mainly because the list of InterWiki prefix is as good as it's going to get, I'm afraid.pages is so incredibly large. Also, since they don't use page names, even if we did process their list it would require a non-trivial amount of munging to deal with them. Using the Wikipedia:
It sounds like you have the selection criteria backwards. Wouldn't the number of Adjunct pages be the limiting factor, so that you just look up the pages onthat actually have links here?
Think about what "look[ing] up the pages on" actually involves.
TheAdjunct is using a version of UseModWiki 1.0 that I (painfully - it's not nice code) custom-hacked to add TwinPages functionality to, in addition to a separate script that runs nightly to retrieve the lists of pages from other sites. I could provide a diff, if anyone wants it, but it wouldn't be a very clean patch. I probably should produce a clean patch and put it on the UseModWiki site at some point.
Update in 2008... this is pretty unlikely to ever happen. Sorry. -- Earle