I think the old Crichton novel, Congo, mentioned using a very low bitrate codec over a digital satellite link. So maybe something already exists off the shelf that one could try over amateur radio.
AFAIK there aren't any commercial codecs with such low bit rates, as there simply isn't the demand. The minimum bit rate of GSM-AMR is 4.75kbps. Some military codecs designed for use over HF are in the same ballpark as Codec2 - STANAG 4479 operates at 800bps. There is some academic research on voice codecs below 500bps.
Fun fact: export of very low bitrate (<2400 bps) codecs has been restricted under the same munitions restrictions as crypto (And also when open source presumably subject to the same court outcomes in the US).
Gotta talk to those subs somehow, those ultra long wavelength Comms aren't exactly high bandwidth. (I'm just making this up but it feels plausible, at least a few decades ago)