here's the code for using XRANDR from inside CS's XF86VM plugin, please note that it is a work in progress, so it's full of comments and print messages :
modified ./pluginsplugins/video/canvas/xextf86vm/xextf86vm.cpp: http://pastebin.com/BdYuWuWA
modified ./pluginsplugins/video/canvas/xextf86vm/xextf86vm.h: http://pastebin.com/mkRzGTE0
modified ./configure.ac http://pastebin.com/R9WgdiDj
I tried moving the code to it's own plugin, but to make a long story short, I couldn't really tell if it was working or not. Maybe someone else can do it.