I have a SDIO WiFi module from a Sipeed Lichee Nano development board. These have the same chip as the onboard WiFi/bluetooth on the PinePhone (RTL8723BS).
On the PinePhone mmc0 is the SD card slot and mmc1 is the onboard RTL8723BS SDIO chip. mmc2 is the eMMC chip.
I inserted the Lichee Nano WiFi card into the SD card slot using the SD extension cable.
It's an odd fit. Since the board extends towards the connector.
It barely fits with the back cover in place.
The version I used is an older version you can find on eBay. The new smaller (black) version is too short. I could not close and lock the SD card cage on the extender on the smaller black version.
The Bluetooth on this chip is via UART, but the UART pins are not available on the micro SD module, so these are WiFi only.
On Manjaro Phosh it showed up as another SDIO device on mmc0.
It loaded the correct driver and shows up as wlan1. It works as expected.
There aren't very many SDIO devices out there, most are WiFi/Bluetooth. This may only be useful if you manage to kill the onboard WiFi. There is the Red Bean 802.11ac SDIO module that should also work if you want 5GHz WiFi.