I've pretty much decided to add an i2c bus using a couple of unused pins on the processor. I have a few bme280 temperature/pressure/humidity sensors laying around from another project, and it will be interesting enough to get that environmental info from my garage.
On the PCB, I isolated the lands for those no-connect pins. I should be able to easily tack-solder onto the pins of the processor board for the signals. For Vcc and Ground, I have plenty of places to tack-solder. For one of the pins, I have to use a pin that is set aside for UART Rx or Tx. I wasn't sure whether those would work for i2c, but since I am doing OTA updates, I think they are available without any problems. I did a test on the breadboard, and it works fine.
This won't be as tidy as bringing it out to a screw-terminal connector, as I did with all of the other off-board connections, but I can live with that. To keep the sensor away from being influenced by the main board heat, I'll have the sensor dangling with a few inches of ribbon cable anyhow.
Hey, still 2 GPIO pins left on the microprocessor board (one of which is the on-board LED). Come on, think! There must be something I can cobble together with that.