>The code that populates the MRU stuff can be found in a table at HOME(7)+"taskpane" but I'll be darned if I can figure out where it's drawing from.

Nope, it's in the resource file.

USE (SYS(2005)) AGAIN SHARED
REPLACE DATA WITH "" FOR "MRU"$id

Now, that method is a little heavy-handed, in that it probably deletes MRU entries for other stuff, but you get the idea. Go into Options/Task Pane Manager/Customize/Data tab to see which IDs the code is specifically looking for and season to taste.