Use Gimp.app approach. They wrapped their application in a shell script. The script launches the x11 terminal, catches the display, and then executes the program ( after setting required env values ) . They also detail how to package it on their website:
http://gimp-app.sourceforge.net/This is the approach I use to bundle gtk with my MAC OS X distro - and the method of launching. I've been told it works on both Panther and Tiger versions of OS X without a glitch.
Calvin