As I’m pulling instruments out of an opera flow-by-flow for appropriate tacets, I wonder if a better way is possible. Currently in Setup mode you can “Remove Player from All Flows” or “Include Player in All Flows” — would it be possible to have an option “Remove Player from Flows where Tacet?”
I suppose I’m rather confused what you’re trying to accomplish here. The whole point of the tacet feature is to properly indicate when a player is not playing and have that be appropriately reflected in the part. Are you not wanting Dorico to generate the tacet notice?
Romanos, the tacet notices belong to layouts, not flows.
If a flow is in a layout but there are no players for that flow, you get the flow heading followed by the text “Tacet.”
If there are players but they have no music, you get an empty staff with a multirest labeled “Tacet”.
“Remove player from flows were tacet” seems a reasonable enough feature to have if you are working on a project with lots of flows with a lot of variation in the instruments used.
Yes, most especially on big XML imports. It’s easy enough to assign instruments appropriately as they’re added when you’re composing or doing note input, but on XML imports I often find myself wishing for something like this while I’m trying to chase down which numbers had sleigh bells or not. 
So, you want Dorico to automatically ‘uncheck’ a Player from Flows where it is currently present, but has no notes?
That seems a useful thing.
In the meantime, Dorico will warn you if you uncheck a Player in a Flow that does have music - so you can be rather cavalier about unchecking, and then Cancel the alert, reduce the range of Flows, and try again.
I find this alert is fantastic, it has prevented me from lousy mistakes. Thanks to the team for having implemented it!