I went this route and did not enjoy the experience. It took a bit of patching to compile due to the changes in the X server from the stock configuration. Once I had it compiled it worked exactly as the existing one did: no mouse support. There were no compiler flags/#defines to turn it on, nothing obviously commented out, even all the drivers for various input devices were still there. It just didn't work. I asked Daniel Stone about it (The Nokia guy who performed the changes) and I got the impression that it was indeed something they disabled so it would always be forced to used tslib; he hangs out on the maemo dev list so feel free to ask him since my memory is a little fuzzy after so many months. Even the diffs for the patches didn't seem to account for the problems. The only thing I can think of is that KDrive (the embedded version of X) can only handle one pointer device at a time. The init file for the X server contains this line:
ARGS="-mouse tslib -nozap -dpi $DISPLAY_DPI -wr -nolisten tcp"

I'm no X-server guru by any stretch but we may end up having to put a code shim between the X server and tslib that accepts input from a pointer-type device as well as tslib and combines the stream of events before passing it to the X server. The rub there is that we'll need to make sure we use the same protocol as tslib or the X server will be very unhappy (I tried to force it to read from a mouse instead of tslib by changing the arguments to the init script and it made the X server crash almost instantly when trying to use the mouse).
The sources for the X server are public and in the repo for the rest of the maemo developer sources/packages. Good luck and let me know if you have further questions.

Larry