Yes, you can play UT99--without needing Classic--even on Intel Macs--and in my experience it works great including all my mods except for certain minor glitches, like blocky smoke trails on rockets.
Installing it may be tricky--I haven't tried that because I copied my installation from an OS 9 machine. Still, even an OS 9 installer may work fine if you have a PPC Mac and Classic. Then you could dump 9 when it comes to actual play.
You need the OS X version of the UT app--which can go in your UT folder alongside the regular UT app. And DON'T FORGET (like I did) to follow the readme and use the special .ini files that are provided.