I have PR #57027 in progress with some fixes to allow the mono subset to be built under native FreeBSD as well as cross-built under Linux.
FreeBSD is currently enabled for CI build (Linux cross-build) under these sections:

• CoreCLR Product Build FreeBSD x64 release
• Libraries Build FreeBSD x64 Debug
• Installer Build and Test coreclr FreeBSD_x64 Release

At the moment, eng/pipelines/runtime.yml doesn't have FreeBSD under any of the mono build sections.

After #57027 is merged, could the owners of eng/pipelines/runtime.yml please investigate adding FreeBSD to the relevant section/s.