Submitted to OSHPark. Ignore the missing ground pins on the 2 headers, those are an artifact of the renderer.
I took a lot of wild guesses about the RF bits. We'll see if it actually works.
Highlights are an onboard 5V-3.3V LDO to keep from stressing the Pi's 3.3V regulator, pads for an FTDI-style UART header for the Pi's console UART, some stuff for debugging the ESP, onboard antenna, the usual U.FL / 0 ohm resistor setup for switching to an offboard antenna, and of course it's a Pi Zero sized HAT with a config EEPROM.
KiCad design files are on the Github on the main project page.