Thanks
I did have a look at the boot codes just after posting... what I lack is what the codes are for the corresponding WMs. You already gave the example for desktop=jwm, what is the one for Xfce4? I would guess desktop=Xfce4?
How would one exit X and startx the other desktop? Is it startx=jwm?
BTW, if I drop back to text mode after booting into Xfce4, there is an error message from the startxfce4 script, "Display X already running on display 0,0" or something like that. Could there be a duplicate call to start X somewhere?