Why would you need another X server for fvwm?
Xomap is X for OMAP chipsets (the N800 has a OMAP2 chipset). OMAPs are system-on-chip chips from Texas Instruments.
IIRC Xomap has its origin in the kdrive project for small X servers for embedded systems.
There's IMHO no point in replacing the X server for just changing the window manager. Have you looked at easy-debian for the tablet? There you'll have fvwm.