It's certainly possible to run Urban Terror using a native 64bit IOQuake3 binary. I've been running UrT 4.1 on top of IOQuake 1.36 x86_64 for a few weeks now because of the improvements in audio support (so I can more accurately pick out the location of other players when I can't see them).
/usr/local/bin/ioquake3 +set com_standalone 1 +set fs_game q3ut4
Cheers,
Toby Haynes
(aka AskQuestionsLater in UrT)
Last edited by Nexus6; 05-19-2009 at 08:06 PM. Reason: Adding ioquake command