Apologies for the late response to this.
For entities this seems like a useful phased approach
I have a daft question that you might have sort of answered in
But I'll ask anyway 
basically I'm wondering if this approach would be transferable to submissions if Project Viewers / Managers could be assigned to groups.
My (probably simplistic) thinking is that if app users / data collectors are assigned to a group for Entities, maybe it would be exposed for submissions? Then that functionality could be exploited to generate a corresponding project viewer / manager group to restrict / filter submissions - either in Central or via the API. And obviously there's the 'elephant in the room' that wants to be part of multiple groups... (I have very sociable elephants)
This would be a different take on my 'idea' Restricting web user access to specific forms within a project which is still a thing I'd like to be able to solve for the QuODK plugin to be more widely useful.