i thought there was a map bug where you could hide inside a wall by the RG on q3dm9, but i don't remember how to get into it. surely there are a million custom maps with various hiding places that get played on the ffa servers... you'd just have to catch someone on here that plays ffa.
A nice hiding place on q3md17 you can reach by dropping down from the SG platform and some airstrafing.
Haha, self portrait.
On q3tourney6_ctf there are two invisible boxes in the air above the red or blue base (not both, just one of 'em) where you can get on top of by rocket jumping on one of the pillars. There are some hiding places in dm11 (the one with the green slime) too (ceiling of the quad room, top of the doorway at the PG side of the MH room, etc.). And on dm15 there is a little room on the left side in the lava with 2 or 3 BFG ammo boxes.