Is this what you’re after, Iain?
I made playing techniques for left and right brackets and moved them into place in Engrave Mode. The larger ones are the same PTs scaled up. See the properties panel.
Here’s the file. It took me a couple of minutes.
L + R round brackets as Playing Techniques.dorico (370.6 KB)
I’ve used PTs a few times in this sort of situation for glyphs that essentially belong to a note. Great that you can tweak their position as you like, and that they move with the note. I haven’t found any downsides yet.