If anyone else has an MSII and wants to add A/C idle up, I documented what I did here:
http://www.tmproductions.com/repairs...p-modification
Short(er) version: I did both A/C sense (so that MS can adjust the idle) and A/C control (so MS can turn the A/C on and off). I built the standard "going high" circuit from the hardware manual, but I found the relay output circuit confusing (I still don't know what the diode to 12v is for), so I just replaced the whole thing with a P-channel MOSFET. I also used a 5v relay board instead of an automotive relay to control the A/C compressor clutch. This required fewer parts, which was nice. To connect to the MegaSquirt I used the DB15 connector, which is pre-wired for 5v and ground out. I soldered header pins to the pads in MegaSquirt and use breadboard jumpers to connect everything together. They fit pretty tightly, so I'm not worried about anything popping off while driving. I used JS5 for input and JS11 for output.
Wiring in the car is just cutting the K/O wire under the passenger side dash, running the mode switch side to the common terminal of the relay board in and the DB15 to the MegaSquirt, and the other end to the normally open terminal of the relay board. I ran a 6' DB15 extension cable from the MegaSquirt behind the driver's seat to passenger side of the center stack, and plugged my DB15 connector into it there. I also used my new 3D printer to make a case that holds the relay and the MOSFET.
And that's it -- I turned on A/C idle up in TunerStudio, everything works just like it should.
I'm tempted to add another relay to turn off the A/C on WOT. Unfortunately, you can't just use the Programable I/O features of TunerStudio with the existing relay, because A/C idle up is doing that, and it will complain loudly if you try -- you need to use another pin, which means another relay. I also considered a "panic open IAC" button to kludge around low idle/near stall situations by forcing the IAC open by another 30 steps, but that's just silly and I should just fix the stalling issues instead.
-- Joe