Well, if you tear up the body for the Kahler, it will be possible to put the Mustang trem back later, sans the middle screw....however, I'd never do it, and I used to be a loyal Kahler user several years back (the Mustang trem, ironically, is the trem that got me away from Kahlers). Before that, I was planning on building a custom Kahler equipped Mustang.
If you want to install a locking trem on a Mustang, and have the money to spare, get a Washburn Wonderbar 2001 trem, those don't require ANY routing at all, just a few holes for screws that would have to be filled later at worst.
As for using the Stang Trem, and setting it up for Kahler type stuff, here's the thread I wrote on it.
http://www.jag-stang.com/forum/viewtopic.php?t=14552
