I don't speak for the developers and I'm not involved in decision making in any way, so keep this in mind...
I doubt they would support this. Android intentionally makes it so users cannot access private storage for apps. If GrapheneOS permits messing with app-private storage for one app, then why not another or another? I feel like it cannot even be considered. That's just my opinion.
I feel like it's not a good idea from the project's perspective. A feature like this would only benefit a small number of GrapheneOS users. It also has nothing to do with privacy or security so it's not something that the project would normally work on.
Again (and yes, I'm repeating myself to make it very clear), these are just my personal thoughts.