I would not expect this to work without issues, as Rust does not support a stable binary ABI across different compiler versions. How can we be sure that the two "codegen backends" will always be implementing the same binary ABI?
What is left is to account for the ABI in any monomorphizations that occur at the boundary, i.e. when your own structures monomorphize generic functions in the dependency.
When the compiler creates this monomorphic variant, and lowers down, it can provide the necessary details of the ABI.
I'd expect the generic code to lower the function parameters to primitive types (pointers, ints, floats, etc.), but the backend would then distribute those over registers and/or stack. Keeping that compatible would still require an (unstable) specification implemented by all compatible backends.
Unwinding might be tricky as well.
Not sure about that. Calling conventions are defined by the platform ABI which is what the backend implements, so any conforming backend should still be mutually invokable. That's why a Rust program can emit an ABI-stable C API and why you can call GCC built libraries from an LLVM built executable. The lowering of parameters to registers is constrained by this because the intermediate representation understands that what are parameters to functions & then follows the platform ABI to lower it to function calls.