It looks like the MCUZone CM4 4G mentioned here [1] is intended for just that purpose! I would be happy to try out a chipset provided it's not too expensive, and (ideally) it has good Linux driver support.

Drivers seem to be the dealbreaker for a few cards, if they rely on PCI express features the Pi OS or SoC currently don't support (like BIOS, or sort-of MSI-X support).

[1] https://github.com/geerlingguy/raspberry-pi-pcie-devices/iss...