This is easy to achieve if, in the first bar of the system, no keyboard percussions or drum kit is playing.
However, if in the first bar of each system there is a vibraphone note or a drum kit ones, Dorico splits the staves and it is no longer possible to have a single “Perc.” label.
I reproduced for you the same situation on a new, simple file with random notes (Dorico file in attachment).
The percussionist on the third line plays 2 instruments: xylophone and claves. At bar 1 he plays the claves, and the staff label is correct (one label for the 3 percussionists).
With 3 three players setup, I don’t see a changing in the bracketing, when the pitched percussion instrument appears (as you said in the OP).
I only see that the Engraving Option Staff labels for identical adjacent instruments held by single players: Group between staves will be broken, when the pitched percussion appears at a system. And for this I don’t see a solution (but someone else possibly do).
Generally speaking I think that your approach may be difficult to obtain (by design ?) because this particular situation is not very convenient: using the (edited) player names (that now are identical despite the different instruments) as staff labels in conjunction with the grouping between staves option, you don’t have any indication of which instrument is playing (no staff label indication, no instrument [change] label, if the instrument doesn’t change at start of system, and no possibility of using percussion labels either, because they work only with unpitched percussions [possibly using a custom text in the Properties, if they are not in a kit]).
So I would suggest another, more traditional approach; mostly to have a clear labelling for the conductor.
(I don’t know if this is an option for you, but I don’t see any other way to overcome your issue, here. Maybe someone else has a better idea. If this is not what you want/need, please ignore this suggestion):
use the original player names as staff label for the first system (), and automatic abbreviated instrument names labels for subsequent systems, and use the Player group label to indicate all your percussions, if desired.*
Here an example of my suggested setup (I added some material to show it clearly in various circumstances):
Screenshot (hopefully enough readable):
Just to be precise: I probably oversimplified my Dorico example to help you focus on the issue, but in reality, with my approach, if each percussionist has more than one instrument — as is usually the case — the instrument changes appear correctly and clearly, except for the first one, which needs to be added manually.
But the labeling problem, of course, remains unsolved.
I’ll discuss with my publisher whether the approach you suggested is feasible for us. Any other suggestions are, of course, more than welcome! And perhaps a future release will address this behavior of our wonderful software? @dspreadbury
I’m following your solution but, fore some reason, if an instrument change doesn’t appear on the first bar of a staff, the group label moves to the right, overlapping the instrument labels.
Unfortunately I can’t reproduce it on a dedicated file and I can’t share my Dorico file, but you have here two screenshots:
having an instrument change on the first staff (Hand Cymbal, used in the previous system, changes to Tambourine):