I see a different possibility, though perhaps you don’t intend to implement that either:
If you had multiple layouts for a given database (entity view), and they could have unique security settings per-layout (which I would think is easier to implement than per-field permissions), then you could simply put sensitive fields (ones that not everyone should see) onto a different layout with special permissions. The likely failure case for at least some people would be desire to have fields that are visible but not editable, depending on permissions, which becomes more field-level permissions than unique layouts. 