After doing some poking around it looks like the answer to my question is the same old story-- if its truly a custom map every player must have it in order to play it. I was initially confused because some maps that appear to be custom can be used in multiplayer without every person needing them-- those are "coordinate" maps (lifted from the campaign map and unaltered) or, it appears, copies of the existing maps that came with the game with "climate" modified but without terrain modified.

Anyway, I guess we'll still need a community mappack :(