Ah, I think I see where’s the misunderstanding. I was thinking of the case where the derivative work is itself a modified version of the orchard crate, in which case I think we agree I can’t distribute the derivative work (which includes the original orchard code!) under dual BOSL/MIT.
Conversely, in the case where the derivative work is (say) a program that uses the orchard crate as a black box… it’s messier. I think it’s OK to license the program code per se (without orchard) under dual BOSL/MIT, as you say. But a case could also be made that because the program code itself contains copyrighted expressive elements of orchard (e.g., function names), distributing that program code even without orchard would infringe on the orchard copyrights. And then a case can be made that nonetheless, that infringement is permissible as fair use. Fun stuff.
Edit: By “distributing that program code”, I mean distributing it under a dual BOSL/MIT license, thereby creating a false representation that would induce others to further distribute the code under just MIT, which would then be an infringement of the BOSL copyright — as discussed above.