I have a multi-flow work. On the first system on the first flow, I want to show the entire instrumentarium for the entire work.
However, in some of the parts I’d like the “Tacet” box to show in the first flow. To make that happen, I un-tick the instruments I want “tacet” from the first flow – but then of course, they do not show in the first system of the full score.
Is there a way to have “tacet” in some of the parts, and still have them show in the first system of the first flow?
Not automatically, but this is something that has been requested before.
You’d have to use a separate Player for the score, and another for the part.
Or you can, as @FredGUnn suggested in an answer to my question here When will "Tacet" be shown? - #5 by FredGUnn overlay a custom “Tacet”-Text over the music. 
OK, thank you all for your suggestions.
The meaning of the full instrument list is normally who plays in this movement, which is why Dorico doesn’t do it for tacet. I would make a separate instrument list page where you can mention things such as “(3rd & 4th movements)”.