I tried something like that, but I could not select open arena from the menu. I can't find an editable oa.game file. All I have is the oa.game folder.
I don't know how old your post is so I hope this helps out.
There's in the gtkradiant path there is a q3.game/ directory, and a games/ directory.
The q3.game file is in the games directory. So what you want to do is copy your
gtkradiant/games/q3.game and modify that as necessary.