I’ve just tried a bit of a workaround I thought could work, but, alas, no dice. I think there’s currently no way to trick Dorico into doing this until the team builds the tools of the next abstraction layer to handle this natively or hands us the flexibility to edit the bracketing manually.
(You can edit your earlier response rather than adding a correction. Use the pencil button to recall the original post for editing. And we all knew what you meant anyway. )
You won’t get brackets if you include piano staves.
As a workaround, you can use 2 horn staves and two trombone staves, then cover the staff labels with a graphics frame containing a white rectangle and add a text frame with the label for the 4 or 3 instruments.
I would hope that we will have more flexible bracketing and bracing before September this year, yes, but obviously I can’t make any cast-iron guarantees: until something is actually done, anything can happen!