Nforce, left 4 dead does not support multiplayer maps, doesnt matter how much we want that capability.
As Biggs said
Yes, you could link it into a message, or post the links on the forums....but chances are NEW people won't bother doing that and get on other servers. Members would but they aren't online all the time either.
Unless Valve, implements a system that automatically downloads maps for you while loading, its highly unlikely GC will support it on their official server.
Also, why don't you organize a night with members for playing custom campaigns only? I am sure the admins would consider using their test server so you all can play on those pre-downloaded maps.