This would be wicked cool, but with available technologies, will be a PITA and will take considerable time and money.
Regardless there are two routes you could go: wired & wireless. Wired would be considerably easier but by no means a small feat. You need to get between the keyboard and keyboard controller, which means the plastic/silver ribbon cable. You would need to either de-solder the compression fitting, or purchase a new one from a business like DigiKey. The exact fitting you need depends on the ribbon cable's width, number of contacts, and pitch (center-to-center spacing) of the contacts.
So you've got keyboard matrix that needs interpreting. Disclaimer: I am by no means advertising for this company. I used to be head of Tech Support for them and believe they are the best in the business for this particular sector of IT.
PI Engineering specializes in USB input technologies. Their new pi (as in the symbol for 3.14) Matrix Boards coupled with MacroWorks3 (their input/output) software would do the trick. After you successfully wired the rows & columns matrix, you would create a custom device profile with MW3. Here is where you configure which switch (key) sends what unicode character. I constructed several custom keyboards this way. Take my word for it, this is a huge PITA, but it is certainly doable. Before I left, they were working with touchpads & the matrix board together, so I'm sure you could integrate that as well.
If you wanted to go wireless, you would have to employ a combination of PIE's technology with a Zigbee or AtmelAtmega transmitter/receiver.
The tunnel is growing much longer, and much darker my friend. Edited by ILOVEPOTtery - 1/15/11 at 5:11pm