The modem audio is not sent over i2s with the SIMCom SIM7100, it is going over a USB connection as raw PCM and is managed by haegtesse. https://source.puri.sm/Librem5/haegtesse
Interesting. Perhaps I can investigate if USB audio is possible with a USB connected 3G module I'm using in a tablet, too. That would certainly simplify some things.
Thank you very much for a lead!