Just an idea but it would be nice to have some kind of verification system for crates and what OS they can be used on. We could even extend it and let cargo warn you if you're about to compile something that is not verified for your system. Since it's difficult for one developer to test all OSs, there could be a function on crates.io where you can confirm that the crates works for you system if not already verified.
Do you think that such a statement of platform support should be based on passing CI for that platform? If so, maybe hooking into the APIs of popular public CI systems like TravisCI and CircleCI would be the simplest option.
We have exactly such a feature in the works! It's stalled out a bit trying to decide on a UI, but I think we're going to try and deploy what we have soon and iterate with feedback.
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CI don't support FreeBSD 
Edit: at least not Travis and I think not Circle either...
Cool!
Buildbot supports FreeBSD. We use a custom buildbot machine for testing nix. Hopefully this feature will support custom buildbot instances as well.
It's not very direct, but you can through qemu on travis:
(it's also not a weird thing, I've seen multiple projects using qemu in that fashion)