I was able to get a ported app running with it at one point. And I think Microsoft took a pass at it more recently: https://github.com/Microsoft/WinObjC
But it appears Darling uses Cocotron so maybe you've already seen how well it works for you.
Something similar can be done with Wine as well as MinGW and other similar toolchains and platforms that target Windows.
The only thing making this hard for targeting OS X is that a) too few people care for ready-made cross-compilers to be available so you have to piece this together yourself and b) there is no open-source reimplementation of the OS X system headers and stub libraries to link against so you have to extract the official ones from Xcode but that again is only a matter of people not caring.
For MinGW that is all there because Windows is a much more popular target - many Linux distros have MinGW packages that you can simply install and start cross-compiling. The same could be done for OS X.