Get the game from Steam: http://store.steampowered.com/app/13240/ and use a newer renderer: http://www.cwdohnal.com/utglr/ (OpenGL) or http://kentie.net/article/d3d10drv/ (Direct3D 10).
Set FrameRateLimit=60 under [OpenGLDrv.OpenGLRenderDevice] in UnrealTournament.ini if the game runs too fast when using the OpenGL renderer.
Use the registry fixer from http://www.pseudorandom.co.uk/2001/paradox/utmodsfaq/#install-umod to install UMOD packages.
Get a prebuilt Wineskin from http://unrealosx.webs.com/downloads.htm
winetricks + Steam + the replacement OpenGL renderer works, or use install_wine.sh and either the OS X download above or original CDs.
The native Linux client is now very old, difficult to install, and doesn't get the benefits of a newer renderer.
After running install_umodpack.sh, umod -b <path to install>, umod -i <some mod>.umod
Downloading custom maps or mutators is really slow by default. redirect-server.js serves files out of your UT install directory over HTTP and modifies UnrealTournament.ini so that clients know where to look (http://<local-ip>:3000/).
Run npm install, then UT_HOME=<path to install> ./redirect-server.js before launching UT.