This is my biggest concern for iOS too. I'm using Rust at work for mobile app middleware - they've already added bitcode as a hard requirement for iWatch, it's probably only a matter of time until they enforce that on the phone/tablet platforms too.
I didn't realise the Apple LLVM fork was readily available - it might be worth investigating what the diff is, though I can't see what the base revision from the LLVM repo is. If this always has to match the live Xcode version this could be a real pain to maintain. Another problem after bitcode support is that the enforced compile options only allow 2 exception handling mechanisms - between obj-c and C++ both are used, so Rust code won't link without panic=abort.
Another idea might be to transpile Rust code into C (or possibly C++ for panicking). It might be practical now that rustc is using the MIR since most of the fancy high-level features have been baked out.