Make a batch file that says this:
ut2003.exe -nosound -640x480 -exec=blah.txt ini=MaxDetail.ini userini=MaxDetailUser.ini
blah.txt should say this:
That SHOULD fix it. If it doesn't, you'll have to set something in UT2003.ini.
Also, you'll have to copy MaxDetail.ini and MaxDetailUser.ini from UT2003\Benchmark\Stuff to UT2003\System.
You won't need to use Fraps either--it'll run the test for your demo, and then dump you back to the main menu. Hit tilde, check the last line of the console for your framerate.
And, change 640x480 to whatever resolution you want.