You can create a Playing Technique of the brackets. Use the brackets in the default SMuFL range in the editor!
You’ll need to scale them down slightly. I’ve used 80% here: perhaps more would be better.
Dorico will place the PT above the mordent, so you’ll need to move it in Engrave mode:
As it’s a Playing technique, make sure that you have a “None” Playback Technique, so that it doesn’t affect any actual playback by triggering “Natural”.