Have a look at Bartel's site, with his scope.exe program you can use ordinary serial mouse (with suitable mouse driver of course. Acceleration and sensitivity must be under control, I am using gmouse driver).
http://www.bbastrodesigns.com/BBAstr...ted_Telescopes
It does not work for mac, though... but there is a source code provided so maybe you can do something about it?
There are many cheap laptop PC's around, maybe it is feasible to think about buying one specially for the purpose.
I obtained my Fujitsu Stylistic 1000 for $50, and it is perfect for the purpose as it does not have built-in keyboard.
And, if you ever plan to motorize your dobson, the half of the job is already done :-)