Hrm, that's specific to the Teensy. I wonder if the same can be applied to a 328P, or whether I'll be forced to build a Teensy in the design with a second USB port. Meh.
Something else popped in my head this morning though ... they say a man thinks best when on the throne ... maybe so. This board design will also have one of either wired or wireless ethernet. Technically it should be possible to have the device show up on the network, and be able to transmit a new data file for it to write on to its SD card, yes? It should all come down to the coding, and how to tell it that you're about to send it a new data file that it should just put on it's onboard SDcard. That same coding should also do its normal stuff, like running the sketch itself.
Thinking out loud, the sketch would be like any other sketch would be, tell the MCU what to do, from receiving data through the ethernet (whether wired or wireless), to controlling the servos. With everything build as loop calls, it should be possible to add a section that says 'if you see this sequence of commands over ethernet, go in this mode' where this 'mode' would be to receive data that goes straight to the SD card (while halting everything else the sketch is doing.) Or just have each unit poll a master unit on the network and if present, pull a file from it.
I'm sure I'm missing something here ...