Oh right, nearly forgot about that.
I guess I was wrong about simply being able to vote for a special custom config. Otherwise you'd just be able to set both the correct maps and settings with a vote for config "ffa_slipgate.cfg". Looks like that can only be done by rcon. You can still vote for the right genflags to change the available classes, though -- just look up the proper genflags in the manual on the website (don't know them by heart anymore, either!).
If you're looking to play a specific custom game, give me a heads-up, and I can set the maplist/config for you.
Here are the demos:
20100413_demos.zip (7 Mb) - 3.5 maps, all my own PoV (It's been so long that I can't even list the actual map names!).