I am strongly disinclined against this feature.
This would break the guarantee orphan rules exist to uphold because it could make it possible for two crates to define the ‘fills_declaration’ for the same trait, making them impossible to build together. Though this requires a serious misuse of the feature by the shared parent crate, it is still something that could happen and lead to an Irreconcilable Ecosystem Split Disaster.
It is a lot of infrastructure & likely very confusing for many users. The gain does not seem worth this downside, setting aside the previous objection.
I think that between specialization and mutual exclusion we can still do a lot to solve this by enabling a greater variety of blanket impls. Until those designs are fully implemented and we know their success or failure at combating orphan issues, I am reluctant to explore new ways to violate coherence.