Curses!

It would appear that the AHRS SPI pins aren't all connected...the SS line isn't connected to anything, which makes it impossible to use the AHRS in SPI slave mode. Looks like there will be some PCB rework after all...

This is pretty annoying. My options are:

1. Continue as planned. Cut and jump on the AHRS board: solder a wire from a TQFP pin to the 6-pin header.
2. Make the AHRS the SPI master. The SS line is currently connected to CLR on the shift register (used for PWM output), 
1. The hard way: connect an AHRS UART pin to the 6-pin header (at least this jump is from a .025" post to another .025" post). We won't be using the UART in normal operation, anyways. Then connect that pin to the Arduino SS.
2. The easy way: connect the Arduino SS pin to ground, so that it is always active. There isn't any contingency if the SPI gets out of sync, though.

Looks like 2.1 is probably the best option. I'll use pin 6 on the SPI header (currently connected to RESET on the AHRS and nothing on the HIB). On the AHRS, I'll need to cut that trace and solder a wire to it from the RXI pin of the UART header. On the HIB, I'll cut the trace and solder a wire from the pin to the Arduino side of the cut, and a wire from one of the extra I/O pins to the shift register side of the cut.

Okay. No big deal. Whew.