it's impossible to use feh or nitrogen to manage my desktop wallpaper if xfwms composite extension is enabled. I also don't have this issue if I use compton with xfwm.
nitrogen + xfwm + xfwms composite: http://i.imgur.com/udAM72H.png
nitrogen + xfwm +compton: http://i.imgur.com/HnfNi81.png
It does not rely on xfdesktop specifically, it relies on a desktop as opposed to the bare root window.
Anyway, this is enabled in git master. You can enable it as well by rebuilding xfwm4 from sources with MONITOR_ROOT_PIXMAP=1 defined (see the ML thread https://mail.xfce.org/pipermail/xfce/2015-May/034467.html)