After long delays, I got a Raspberry Pi Zero and have wired up some hardware for testing. A pl2303 usb-serial converter gives me a console, and a usb-ethernet adapter is hooked up through a otg cable. A clone of the adafruit ina219 breakout is monitoring the "12v" input to the dc-dc converter powering the pi. An example python program is giving reasonable results, so I'll need to figure out why my perl code isn't.

Current plans are to switch to an Orange Pi Zero with built in Ethernet and Wifi, multiple USB, and multiple serial ports. Unless I do my own PCB, I'll use one of the ina219 breakouts with the R100 resistor removed.