Yes! There is no obvious reason why it shouldn’t be possible to unwind across languages in a multi-language application.
Of course any valid implementation would always have to make sure that destructors are run properly and frames are unwound correctly. This is a problem, but not a very hard one - as Rust uses the C++ unwinder right now, it already has some basic compatibility - it’s just not guaranteed and if you try to catch a C++ exception in Rust, the runtime gets very confused (it expects a Rust panic but there is none).
Yes, unwinding from Rust through FFI boundaries (or vice versa) is undefined behavior right now. But an RFC can of course change that!