So, I had a realization earlier... the USB interface on the Nano DCCduino is actually on a separate chip. On a hunch, I plugged it into the breadboard to see if it would light up if I powered it externally...

Well, then. That makes things easier.

So, in light of this... I'll be switching back to using the Nano for this project. (more pins makes a lot of things easier, and I won't need to worry about weird DigiSpark-specific library ports) I just need to solder the ICSP pins on and hook up my BusPirate, and I'll be back in business.