This is just to say that I haven't forgotten about this thread, but I am snowed under with end-of-semester chores for the day job. I'm hoping to have time for proposal draft V2 next week.
To respond to one specific point that several people raised, I don't like a Rust-private copy of the environment "even if you know that the consumer is in Rust" because that means that whether or not the consumer is behind FFI becomes part of its interface contract.