I'm thinking....you probably wouldn't even need a control IC. In this application, you're basically just taking the sensor and using it as a switch. It's not like a touchpad mouse or an ipod control ring where you actually need location coordinate data. The wiring should be a piece of cake too. You could just yank the IC from a regular keyboard and trace the...um...traces of the old membrane sheet to get the IC pinout then hook up the cap sensors as switches.
Thats kind of what I had in mind (except I was thinking about just creating the circit path that is mapped out on the membrane with suitable wiring, and then just wire it in to the stock circitboard)
I was thinking about just hiding the switches as part of the Alpha-Numeric set-up, instead of having to press the keys together. (Think touch sensitive lamp, rather then typical cheapo keyboard)
From my own keyboard modding experience, you might want to use a donor keyboard that's as old as you can find. Rather than using a modern USB keyboard, you might save yourself some headaches by using an older, PS2-plug keyboard, then using a USB adapter with it. The reason I say this is because the ICs in older keyboards are much larger than newer ones and are so much easier to solder to. The pins are nice and spread out...and quite sturdy. The overall size of the IC is still small enough to hide, though. The newer keyboards use mini-circuit-board with all micro-surface-mount components which are HELL to work with.
I know what you're saying with the keys...you just want a completely stealth setup where you just have to brush your fingertips across the letters on the faceplate to get a response, right?
What I was trying to say about tracing the membrane sheet was....ditch the sheet entirely and save the IC...but keep the membrane sheet around because you can use it to map out the connections into the IC.
The way the IC works is pretty simple...you might know all this already, Lurker, but let me explain anyway for the benefit of anyone else reading...you have two banks of pins, one on each side. Stored inside the chip is a table of information that maps connections to the pins. Basically its like...if you touch pins 1 and 12 together, you get a 'Q'....if you touch pins 5 and 23 together, you get a '7', etc. (pulled those numbers out of my ass, btw). If you look at the membrance sheet, you can actually trace the 2 lines from each key, back to the pins they are connected to, then you know how to wire up your switch to make that character. Some of them are shared, too. You're not likely to need to press 'scroll lock' and 'F8' at the same time, so those two keys might share a pin.
With the capacitance sensors...because you're just using it as a digital switch (on/off) and don't need any location data, it might wind up being just as simple as taking the positive wire and running it to one pin, and the negative to another pin...done (though you might possibly need to adjust the operating voltages with a resistor or two).