I think I understand the way Dorico approaches this—when you set a local property as global, it moves to the global settings and that item can no longer be changed locally, without undo-ing the global property.
My current approach to dialogue in theatre score, where it needs to appear on the Score, the Piano/Vocal, and the Piano/Conductor, is using staff-attached text on the top vocal line, when available, or staff-attached text on both the rehearsal piano staff (Piano/Vocal) and the Piano/Conductor staff when there is no vocal. This is fine, but in the latter scenario, I need to hide one of the dialogue texts on the score, and if I have to change it, I have to change it in both places.
Slightly more ideal would be to have it as a system-attached text, where I can then hide the dialogue (a local property) with Set Local Properties: Globally enabled, and then be able to un-hide it in those two part layouts. If I use system-attached text now, I have to hide it locally in 10-20 part layouts.
Another approach to this would be group-attached text, which might have use for some very specific use cases but probably not enough to justify the whole can of worms. The wildest option would be having “locally/globally” be supplemented by an optional popout window with a checkbox list that allows you to specify which layouts received that change in properties and if it would be default for any new layouts created. Sounds like a Dorico 12 feature.
I don’t expect any of this to take priority, and the current setup works fine, but I couldn’t find any discussion of this question so I thought I’d raise it in case it’s something others could benefit from as well.