The GHGSat constellation's payload software is still mostly OCaml, although a limited amount of newer from scratch components are indeed in Rust. It's been working well and on 16 satellites now - but as you said the main challenge has been training developers to Ocaml and I doubt they would write new code in it now.

Basically from yocto have a "rootfs" which is your standard image + gcc, use it to build a docker container or chroot in it, add qemu-arm-static, build the OCaml compiler from opam and then use it to build binaries. Not the most straightforward, but it works... fine. And in the end it's just a docker image like any other that is used to build our binaries.

With that said this has been running for a few years, I didn't check if upstream OCaml cross-compiling made progress since then.