Probably just making Retro68 target LisaOS besides Mac OS would be an easier and faster to achieve target.
I tried them and it's impressive to cross-compile for Mac OS 6 a C++20 application.
Main thing is to figure out the following:
1. ABI of the LOS kernel, this is abstracted out in the Pascal libraries and not fully documented in the developer documentation. The calls themselves are listed in the LOS guides, but not the binary interfaces. There's a lot of weird traps, some are A-Lines, others are TRAP and so on. Hard to say how it all works. Perhaps the header files offer clues, I don't know. Don't know if these differ from LOS 1.x, 2.x to 3.x.
2. The format of the actual binaries themselves. Usually these are .obj, but that extension seems to also be used for the pre-linked .obj outputs of the compilers, so...
LOS source code release might help with this, if not reverse engineering is all we have.
I've had the wild idea (or delusions if you will) that at some point we'll get this resolved, so I could build something like the AMS emulator, but for the Lisa, where we could remove the actual LOS kernel and run native Lisa apps on Linux and other on top of modern OSs - if we can. This would be my goal, for something like LisaEm 4.0 (but, that seems non-possible at this point in time for other reasons.)
Something like Arros or other OS's, and a modern document oriented desktop system on top of something like LXDE or something else.