I saw this link and wondered if the information may be of some use?
http://www.left4dead411.com/forums/index.php?action=vthread&forum=9&topic=8755
Ok maybe we don't want to go the whole hog and delete the non-versus maps but some of the stuff sounds useful, e.g.
a) Get this plugin: Force Mission Changer (http://forums.alliedmods.net/showthread.php?t=81982) In Versus Only at the end of a versus mission, the server boots everybody off the server and goes back to the first map on the same versus mission. This plugin resolves that issue. It allows everybody to stay and and it switches to the next versus mission.
I'll have a look at this and check it out. Don't want to lose the ability to do co-op on demand though :)