As I previously noted, I had to make a small change to get the javascript text selection thing to work on this website. The change I made seems to be addressing a symptom, rather than the underlying problem, since when I look at the source code it should work as written. But anyway, I made a vpatch for the MP-WP tree, so if anybody else sees the same problem they can use this fix.
mp-wp_fix-textselectionjs.vpatch (text version)
mp-wp_fix-textselectionjs.vpatch.peterl.sig (text version)