The stylus of a tablet PC should essentially BE a mouse as far as the game is concerned. There's no tablet specific interface code or anything like that... WW gets its mouse XY coordinates from SDL which gets them from the OS, and that's that. It appears that it gets broken somewhere, most likely in SDL if other (non-SDL) programs work fine.
After some searching I found this little tidbit:
http://www.nabble.com/Re:-SDL-and-ta...-p2681303.html
It seems that it hasn't been fixed in a stable, public release of SDL so far (the latest release is from last fall, before the post above). Hopefully it will be soon, as WW would certainly play just fine with a stylus.
Btw, when playing in a windowed mode you can type in any values for the width and height in the launcher. So if your screen is 1024x768, anything less than that (so it's still usable) will be fine.. Try 960x704 for 32-pixel margins.