Current situation: 

there is no easy option to add a custom server-side check for permissions. There is the role-based options for a screen:

but for particular use cases (e.g. checking, if a user has a role AND belongs to a particular group), there is no easy option. One would be to use custom Fetch Data from Other Sources server Action, make a check, and then fetch the respective Aggregates. But it leads to losing the asynchronous ability of Aggregates, since they are not called async.

The other option would be to make the respective check in each of the used Screen Aggregates, but then we end up with a lot of code repetition.

Suggestion:

To have the option to call a Server Side Action for custom server-side check for permissions for a Screen. This would be loaded on the same level, as the Role check and before the Screen is loaded.: