I'd forgotten to specify layout-compatibility, so I just added a small section in that regard.
The forum also helpfully pointed out two earlier threads, Implementing traits and types for multiple foreign crates (pre-RFC) and Pre-RFC: pub(crate) impl Trait for Type, which might be good to link here for reference. I'm pretty sure my draft avoids the concerns with the former, and it does support the latter's use case as the scoped implementations follow item privacy rules.