Just a little bit of progress to report so far.

I received my Arduino kit and some controls the other day, and put together a quick mock-up of the code to handle the control input and send it over the serial interface to the empeg. I've got the knob left/right/press and the keypad input working with some cheap controls that I won't be using in the final input panel (if I get that far.)

Thinking about it more, I'm coming around to the notion that hard-wiring a Kenwood/RioCar remote will probably look lame, so I'm going to give that mini keypad a shot. I will still need to find some buttons small enough for it, or some sort of plastic membrane style thing to put over the switches, but assuming I can figure that out, that'll probably be a nicer looking install. Another option is to do my own matrix of square pushbuttons, which opens the possibility of using LED-illuminated buttons, but the ones I've found on Digikey and Mouser are all pretty pricey.

With all of the inputs, I'm out of pins on the Arduino, so I'll have to do some muxing. It looks like this guy is a cheap and easy way to get as many as I'll need.

For the input panel itself, I found a reference to this site in an old thread here, and it looks like getting a panel in anodized aluminum will run me somewhere in the $30-$40 range depending on how many drill holes/cutouts there are. Seems reasonable, but it's the kind of thing I'll really need to get right the first time, as they won't do proofs for a one-off project. I'll probably just be using wood and epoxy on the back of the input panel to mount the controls in, as a full enclosure would just end up being bulkier and would be hidden from view anyway.

The CAN bus Arduino shield should be here early next week, and I look forward to playing with that to see how much control I can get over the factory system and Lockpick, which should also be here next week. I also ordered one of those quickconnect harness jobs so I don't have to hack up the factory or Lockpick harnesses.