The OS could provide a whitelisting API and an "app picker" to the user. The password manager app itself need not know the destination app, the OS could manage the binding. The password manager could invoke an OS API to create each association, with user-OS input for app selection. This whitelist approach would be similar to SE Linux or other mandatory access control systems.

This kind of inter-app policy could also be useful for opening URLs, e.g. open all URLs from untrusted app A in Brave browser where Javascript can be easily whitelisted on a per-site basis.