@nephros kindly built a test installation of mosh, which is currently available for aarch64 with:
ssu ar mosh-testing https://repo.sailfishos.org/obs/home:/nephros:/devel:/mosh/sailfish_latest_aarch64/
pkcon refresh
pkcon install mosh
It has also been submitted to chum:testing, so this repo will at some point go away. I’ll mark this down as an answer, thanks again to @nephros!