Did you take a look at the project that @charles_piano posted above? The percussion part uses a custom template with two music frames, each one filtered to a particular flow – the first frame shows just the Instruments flow, and then the second frame shows the main “Flow 1”. In the score, the single music frame has likewise been filtered to show only Flow 1.
Setup mode can be a little tricky. You can select a Player, a Flow, or a Layout; when you do, that entity turns bright blue, and the other entities to which it is attached display checkboxes. So when you first create a flow, and the flow is selected, it is bright blue and the players and layouts it’s wired up to have checkboxes. At that point, you can remove the flow from the score by unticking the box on the Full Score layout.
Here’s a version of the file provided by @charles_piano which I think also accomplishes what you want, without filtering frames. I first slid the Instruments flow in Setup mode so that it appears before Flow 1. Then, with that flow selected (bright blue), I unticked the box in the Full Score layout, removing the flow from that layout. Then I went into Engrave mode and removed the page template override and the frame filters, since in this configuration the default page templates work just fine.
If there were other players in this project, you’d want to make sure that the Instruments flow is not wired up to any of them. You could do this by selecting all players at once, except for the drum set, and then unticking the box for the Instruments flow.
Showing assigned notes for percussion (asherber edits).dorico (541.0 KB)