Here's where I'm at right now.
I just tried Firmata 1.30, applying orphans' suggestion of including pins_arduino.h. At first, it didn't compile and I got the same error, but it worked after I poked around and changed the method calls in lines 164 and 165 of Firmata from "port_to_input" to "port_to_input_PGM".
I'm using pyduino.py modified in the following ways: I added a six-second sleep and self.iterate() to init, and set the baudrate to 57600, which matches the speed set in Firmata's setup() function.
But I'm still at the same point: Python can connect to the board via serial, pin 13 verifies being set active, I can get the version number, but any attempt to write a 1 to the pin does nothing. The rx light flickers, but that's it. 
Mellis: Thanks, I'll try using that Firmata and let you know how it goes. No, when I upload a regular sketch (like the hello world led test) the board starts the program only a couple seconds after the transfer is complete. I put a six-second sleep in pyduino.py so that no data would be sent until after Firmata does its initial pin13-version-blink thing. Thank goodness my Arduino doesn't suffer from bootloader issues!
Thank you both again for all your help. It just goes to show that open-source communities have more than one advantage over the big guys. ;D