well, you could upload 1 or 2 campaigns every month, and keep only 3/4 custom campaings in the map rotation at once, along whit the oficial ones of course.
this would give time enough time for players to download the campaign and get to play it. and late newcomers wouldnt be completly alienated from the server neither.
also, have some info in the !motd about the custom campaigns in rotation and how to get them.
by the by, might just be my machine, but !motd has never worked for me, cant scroll and clicking the links doesnt do anything at all.
"the way custom content is supported for the game is garbage"
i agree, so hard.
the only way it helps, its the download-link provider on the server selection menu.