Hmmm. For what it’s worth, I did an arrangement in B major, and the Bb trumpet parts automatically showed as Db major, without me doing anything. But just now I transposed it to E, and the trumpets showed up as F# major. So I don’t have an answer, sorry…
In the full score, click on the first note or rest in the trumpet part, type shift-K to invoke the key signature popover, type Fb, and then type Alt-Enter. This will change the key signature of just the trumpet part.
I don’t think Howard’s tip will help either, unfortunately, because you can’t create a key signature that is confined to a single layout (it will appear everywhere, even if you create it with Alt+Enter) and in any case you can only currently specify the key signature in terms of the concert/sounding pitch, so you would have to specify E major again, and you’d still get F# major.
I’m afraid to say I don’t think there’s a good solution for this at the moment. I think you’ll end up having to export your trumpet layout as a separate file and then transpose it up by a diminished second to change everything from F# major to Gb major.
I had a similar problem with regard to Gb/F# and guitartists
In a post I made I wrote…
I am writing a piece which is in Gb major. I have written the guitar part as normal but through experience of working with guitarists; many if not most, prefer to read sharp keys instead of flats. I have copied the guitar part, created a new F# maj key sig just for that part (using Shift K then 5#, Alt enter) and changed the notes (using transpose by dim 2nd and unflagging 'Keep relative respelled…).
There may be some steps here to give you ideas…not sure it would work 100% though.