I got the board for the first revision of the slave module board in the mail yesterday. I assembled it it today, behold!
And best of all, JTAG programming worked first try!
The only weird thing was that the LEDs were on when I connected power. Which was odd, the BJTs controlling power through them are connected to the MCU, which I assumed would put 0V on every GPIO until told to do otherwise. So that'll be task number one in troubleshooting the device. Overall, today was a very good day.
This is the schematic, if you're curious: