I found out that it wasn't actually that hard to get the emulated USB tablet working inside my SuSE Linux guest.
I googled a bit and found what I needed to build this InputDevice section for my xorg.conf:
Section "InputDevice"
Driver "evtouch"
Identifier "Tablet"
Option "Device" "/dev/input/event3"
Option "MinX" "0"
Option "MinY" "0"
Option "MaxX" "32767"
Option "MaxY" "32767"
EndSection
I haven't been able to get it going for my FreeBSD guest yet. It looks like it wants xorg-server.h which will not be present before X.Org is updated in ports to version 7.x.