I have used Tillanosoft's Pocket Tweak for this sort of purpose for a very long time, to back when it was called PalmTweak (ew, ick, sorry...). It's a little control panel (settings) applet, which appears after installing and soft resetting. You'll need to install their tGetFile.dll too if you haven't already, to enable use of the program/file association thing if you want to use that. Freeware, and darn nice stuff too. One caveat; make a thorough backup before trying to make any SIP besides Resco Keyboard Pro, or the native Letter Recogniser or keyboard into the default IM. If you choose Transcriber for instance, after a soft reset the device will not be usable. So backup, then make the change in this applet, then test it and see if you have to hard reset and restore. Anything like this which is system-wide and loads on boot is a bit dangerous.
http://www.tillanosoft.com/