you can try using "VJBrisk". It is a simple app that can redirect a hard coded call to an exe to another EXE. Here is the xda thread for it:
http://forum.xda-developers.com/showthread.php?t=477519
Basically you download VJBrisk.exe, rename it to tmail.exe, then create a shortcut to your preferred msg app, name that to tmailLNK.lnk (do all this in /windows). Then when piont UI executes the renamed VJbrisk, vjbrisk will open the shortcut to your preferred app. I had used it in the past to force M2D to open gmaps on the storage card before we had cooked roms on the i910. (when device memory was at a premium...)