Anyone can vote for maps on [DARK] (/callvote map <mapname>) and we have that map installed on the server. If people want to play that map they can simply vote for it. This way we've already been playing lots of different custom maps in the past few months (even rather unknown ones like Tatooine_Outpost_2_0 with 24+ humans). However we're not going to switch the "default map", cause we don't want to exclude some players (who don't use custom maps or jk2mv) for weeks.
As usual, Boothand posts shortly before I do (I should stop replying to chat messages while writing posts or before proof-reading and submitting them).
One little thing Boothand's post reminded me of - as far as I remember [DARK] automatically switches back to a default map when the server didn't have any human players for 15 minutes.