I've just been experimenting with running x2go, and when I tell the client to run fullscreen, it apparently tells fvwm "don't manage me", which works OK till I use the x2go hotkey Ctrl-Alt-M to minimize the fullscreen client and get back to my normal local desktop.
There doesn't appear to be any way to restore this un-managed client now. Has anyone already figured out a way to best operate fullscreen x2go under fvwm? Tell it to grab the window anyway, but put the client window at 0,0 with the decorations off the screen perhaps? (Is there an easy way to refer to the position of the window inside the decorations in fvwm?)