I have an MS II, so I had to build a new input and output circuit (which is described in the hardware manual, and wasn't that difficult), and I decided to use the DB15 for the external connections. I'm also using one of those Arduino-friendly 5v relay boards instead of an automotive relay to keep the control circuit simple.
I was going to try to have MS automatically turn off the A/C on WOT, but it seems that I'm not allowed to have A/C Idle Up and a programmable output use the same pad. I could wire up a second output pin and another relay or some other logic in the A/C power circuit, but I realized I don't really care quite that much. My custom UI will also be reading a bunch of state from MS over CAN bus, so I can have that turn off the A/C instead if I like. A/C idle up also removed a relay from my custom UI plans, since I can just pass the 5v signal right from my Arduino to MegaSquirt's pin.
-- Joe