Unfortunately that does not work, this is not a Gentoo/Linux specific thing. UT2004's server software has a hard set max transfer rate for custom content. What you need to do is install apache (recommended to use another server, not the one the game is on, to decrease the load), uz2 compress the custom files you have, and put them in a directory in apache users can download them from. Then in your UT2k4 ini file, set up the redirect to point UT to that folder, then when users connect it downloads it from your apache instance instead. This is how ALL UT2k4 dedicated servers work, I host/admin a few myself. You can find more info on this at
www.unrealadmin.org .
If you need any help setting it up, let me know, I've set up countless 2k4 servers and have hit about every bug and annoyance possible.
Also, since you're into 2k4, here's a shameless plug for my sites, they're not fully rebuilt yet since i migrated to my new host, but I keep my UT2k4 maps and mods here. (not all of them listed, uploaded yet)
http://www.midnightfragfest.com and
http://www.raptor85.com