Dorico Pro 4 User Here
I am making an arrangement for choir that will have a sudden modulation from D-flat to A Major. However, the choir will be singing a D-flat as it modulates. It is customary to notate it as a C-sharp after the key change to make it make sense, however, due to the level of the choir that will sing the arrangement, I need to indicate somehow that they will stay on the same note. I’ve seen Finale-user Colleagues put a D-flat in parentheses next to the C-sharp, which I know I can do that by notating a different voice and then make that note appear with parentheses around the note head.
The issue with that is the parentheses for some reason also surrounds the C# note in a different voice instead of the D-flat note. I have no idea how to make it only surround the D-flat note. I attached a picture to show what I mean.
I need help.
A couple of alternative options (that might result in neater, more spaced music anyway) are:
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Input the Db at a later rhythmic position (e.g. a quarter note after the C#) or as a grace note
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Use a trill with a diminished 2nd interval, set to hide the trill mark, and use the auxiliary note appearance (excuse the strange image error on that page!). This produces a small, stemless, parenthesized notehead to the right of your C#.
I always see this type of courtesy enharmonic before the key change, not after.
That’s probably why Lillie suggested the grace note (with hidden stem?) in her first possible solution.
But she didn’t mention putting the grace note before the barline.
To be honest, I can’t remember off the top of my head now which one I’ve seen more of – only that putting the alternative on the right disrupts horizontal spacing least (it’s an “easier fit”). Sometimes I might also just not know something or have limited time to write a response 
Time to remind myself of what Gould says:
Yes – Notice in all her examples, the small notehead – whether an equivalent of the previous pitch or the next pitch – is always between, i.e. before the note with the enharmonic change rather than after, providing the bridge. This is why I think the trill workaround (as you illustrated it, on the new note) is not a good solution.
In the OP’s case, I want to see the C♯ before the key change, before the barline, so it offers advance notice rather than a gotcha.
Now I realize perhaps we have understood the OP’s example differently. I assumed the D’s under the p marking are D♭s, otherwise why would they need a courtesy enharmonic at all?