Finished up the dash wiring stuff. The important news is that the UpRev map-switch buttons work. Basic details for those looking for a similar solution lacking a stock steering wheel:
If you look in the 2009 Service Manual, section EC (Engine Control), the terminal pinouts for the 3 big connectors on the bottom of the main ECU starts on page EC-523. Way down that list on page EC-532, you'll see an entry for pins 101 (SB) and 108 (Y) as the "ASCD steering switch". These are the two wires that run to the cruise control button cluster on the stock steering wheel. SB is Sky Blue, and Y is Yellow. I didn't really try to map it from the pin numbers, I just looked for wires of the correct color in the correct sub-harness that were running over towards the steering wheel area and tried those and it worked, YMMV.
Once you have those two wires, the way it works is the ECU expects a resistor across those wires. The exact amount of the resistance changes depending on which cruise control buttons are currently pressed, which is how it tells them all apart with only two wires.
Back on page EC-437, you can see a diagnostic table of the expected resistance values when various cruise control buttons are pressed, which doesn't quite tell you everything you need to know, but it's part of the puzzle, and you can get the rest by using an ohm-meter to probe around on the stock cruise button cluster a bit.
The bottom line is this pic, which just might be the ugliest drawing ever (but hey it took 5 minutes):
For those unfamiliar with basic circuit diagrams as drawn by 5-year-olds, the squiggly lines are resistors, and the two angly things crossing the center are the two pushbutton switches. The bottom switch is the one you hold down, and the upper one is the one you hit 1-5 times to select a map. If you can read my shitty diagram and assemble that correctly, it should give approx 3960 ohms with no buttons pressed, 250 ohms if the top button is pressed (or both), and 660 ohms with just the bottom button pressed.
You can order very close resistor values online at mouser.com, but honestly it'll probably work if you find something within 10-15% or so at RadioShack, too. It doesn't matter which way around you hook up the SkyBlue and Yellow wires, there's no polarity to this sort of thing. Obviously, the little arrows at the top of the diagram are where those hook up at.