Bulk change Agile Board 'Can View and Use' permissions

We assign all users to a group that has a custom role modeled loosely after the 'observer' role.

The problem we're seeing is that even though they are granted this role on all projects, that doesn't make them a member of that project's 'team'. So when new boards are created with the default project-based 'can view and use' permissions, only those members of the project team (i.e. those with developer role or greater) can see the board.

After trying to grant this permission in the Hub UI, I settled on just leveraging the YouTrack API to change all the board permissions to target this group instead of being project based. Unfortunately, based on the parameters returned by a GET request to the agile board endpoint, that doesn't appear possible.

Has anyone figured this out?