well, I had a working solution once (currently unused). If I remember correctly it looked it likes this: starting X without "xinerama" gives you 2 seperate display. One with dri and one without. I used shellscripts to set the "display" enviroment variable (since pd doesn't/didn't take that option). one to start pd (and its librarys) on display 1 (with dri) with the guicmd option calling another script opening the gui on display 2. The Gem Window now pops up on display 1 (with dri) additionally I ran different window managers (and screen solutions) on each display. First configured for gem (zero border, titles,..) and second for the gui.
hope that helps.. never really ran or tested that "hardcore" ..
m.
I don't suppose you could post/mail the relevant scripts? The discriptions helps somewhat but the scripts would safe a lot of time
thanks
Gerard