Go into the console, while in game.
I have not tested all of these, but they should work. Note that you can use these in multiplayer to play coop maps! But they are buggy and you get stuck a lot. The host of the game seems to be the only one who can play for awhile without getting disconnected over time. But it's cool to mess around with.
Thanks to Nope200, Deth Metal, Oidar
for helping me with these!
*May contain spoilers*
Regular MP Maps
(Note: Assault and CTF are just recycled with the base platform models for the flags):
DM_Siege ("Arena A17")
DM_ZeroG ("Arena G9")
DM_Hangar ("Hangar") *
CTF_Siege ("Arena A17")
CTF_ZeroG ("Arena G9")
CTF_Hangar ("Hangar") *
* = Requires the 1.0 patch be installed to work
When playing Coop I don't seem to have much luck unless it's Deathmatch mode, but your mileage may vary. With tools maybe we could modify it to make it more possible, if the dev team doesn't fix it for us with patches (Rune and Unreal eventually had coop support). Shows what might be possible someday anyway!