@timbray have you seen https://socialhub.activitypub.rocks/t/fep-07d7-a-custom-url-scheme-and-web-based-protocol-handlers-for-linking-to-activitypub-resources/3588/, and the proposal it links to?

There's a bunch of other discussions there along similar lines, eg https://socialhub.activitypub.rocks/t/possible-fep-proposal-cross-android-app-way-to-open-object/2985/14