I suspect you will do well to consider one of the firmware solutions so as to be able to map yer keys to matrix easily - I tried out mnemonix's code and it it worked for me. Not certain when it will be supporting the trackpads and nipples, though. If yer a coder, then this might be some decent fun, if not, that X-thing mentioned above might support more devices.
For just the keyboard matrix, the open-source diy solutions are pretty cheap. Alternatively there is the teensy++, but we haven't ported full membrane keyboards to there yet. I'm going to be attacking that problem in the next week or so, so depending on how fast your project is moving we might get the teensy support to you in time. I don't have any trackpad/nipple devices, so I am not likely the ideal person to do the support fer that. I do know how to mix multi-device into the HID, though - so we'll need to see what happens, I suppose.
Anyway, awesome mod, love do see folks do a great job on the physical layer.