Here's the schematic currently. No display yet, but you'll see that the upper left corner includes a USB header, a boost converter, and a P-MOSFET for selecting between them. I took this design from an Adafruit Feather board (credit where credit is due), so hopefully it'll work. I haven't tested it yet, since I don't have a P-MOSFET with a low enough Rds. Another problem: The boost converter I selected is a pricey chip from Linear Technologies. They're $5 each - as much as my Raspberry Pi!