As I'm sure you can all probably understand, this is a long shot, but if they're mandating stock OS for the app, there's not really much that GrapheneOS can do, and what little it can do will eventually stop working permanently without any bypasses the moment things move to hardware attestation.
That said, the app can support GrapheneOS while using these APIs, by following these steps:
https://grapheneos.org/articles/attestation-compatibility-guide
Like I said, this is a long shot, but that would be the next step here.