make an applescript that calls this command: /Applications/Quake\ 3/Quake\ 3.app/Contents/MacOS/Quake\ 3 +set fs_game "Q3UT" (or whatever the mod folder name is for UT).
Save the script as an application and stick it in your Dock or wherever you want, and then it should open UrT directly.
No. That's telling the finder to perform the command. I'm not sure what the applescript command is for it, but there is a command to call a "command line" of some sort, or something like that. It would be something like:
...not sure what the "call command" is though. It will work under all versions of OS X, so having 10.4 isnt an issue. Try opening up the terminal and entering that command such that it points to the Quake3 application (within the Quake3.app folder) and has the +set variables set properly. It will start UrT directly.