There have been a few posts here asking about how to create accidentals with square brackets, to denote editorial markings. I’ve found out how you can do it! In the Key Signature panel, under “Tonality System”, click on the pencil icon to edit the Tonality system.
Here, you can then add a new accidental with the Plus icon.
In the next dialog, add each of the symbols from the options on the right hand side. You can then select them using the “Previous” “Next” buttons. You’ll have to adjust the position of the brackets using the X and Y values below, and annoyingly, all the chars that were added after the current one inherit its values. You’ll get the hang of it.
Don’t forget to add a value for pitch delta in the top right. Sharp +1, Flat -1. Natural 0.
You can then save this modified tonality as a default, and so it will be available in all new documents! I haven’t found a way of adding it to existing documents without doing it from scratch.
All credit for this tip goes to Jawher Matmati on the Dorico FaceBook group. But it’s such a great tip and really opens up what I can do with Dorico.
This is cute, but I’d really like if this were an actual property, just like it should be possible to place them above the staff. Give editors their due!
You could make your own Playing Technique that shows the accidental above the staff, and hide the ‘actual’ accidental…?
It may be that square brackets will one day be in the same drop down menu of the Properties panel as Parentheses.
And brackets for editorial dynamics. But Dorico has spoiled me…
Here’s a very quickly knocked up dynamic and Above-the-staff accidental, using Playing Techniques.
Very nice. And using the playing techniques editor gives you full control over the spacing between the brackets and the dynamic. Now if only we could export libraries. That feature gets me excited. In a musicologist-nerd way. 
You can save all these things as defaults to appear in all new documents. There may be some XML magic to get them to appear in existing documents.
This is a genius workaround.
Ooh. A word or warning: adding extra accidentals to the Tonality system can cause Dorico’s handling of enharmonics to go a little funky. It’s nothing that can’t be overcome, but worth bearing in mind.
It’s no good. I can’t fathom how you’ve got dynamics in square brackets. And in any case, why doesn’t it work if I just type [mp] in the Shift-D box? The text just disappears if I do that.
Read again… he used a custom playing technique. Not the dynamics popover.
Oh yes, I realise he did something quite different. That’s what I can’t fathom. But as a secondary question, I don’t understand why typing in [mp] into the pop-over doesn’t work. Is it that using cancels it out? Or are we only allowed to use a Dorico dictionary of dynamics?
Dorico does interpret what you type into the Shift+D popover, yes, as you can tell by the fact that if you type e.g. “mp sub” you will get a different result, taking into account not only that “mp” should be rendered as a bold italic dynamic but also that your Engraving Options may be set such that subito ends up shown before the “mp” marking, and with a full stop automatically added.
If you find the popover doesn’t understand what you’re after, you can always add “[” in the Prefix field and “]” in the Suffix field in the Properties panel.
Just to update some comments in this thread. Dorico has now acquired some pretty decent capabilities for Editorial markings:
Accidentals
Editorial accidentals created in the Tonality System no longer cause problems when changing enharmonics: this was fixed in an update. However, you still lose them if you transpose, which isn’t ideal. (Typing in loads of editorials and then deciding you want the whole piece up a minor third…!)
Accidentals above the note, as Playing Techniques, are probably the best way to go for now. They can be accompanied by hidden ‘real’ accidentals for playback.
Unfortunately, accidentals can’t yet be scaled separately from the note for small accidentals.
Dynamics
Dorico provides curved parentheses around dynamics, which is probably the easiest method: Dorico knows it is a dynamic (unlike faked Performance Techniques); and you know it’s editorial. If you really want square brackets, Daniel’s Prefix and Suffix is the way. Though of course, you’ve got to propagate all these through the parts.
Notes
If you want brackets above a group of notes, you can use the Notehead Editor to create a set that includes brackets of any sort, and then apply them to the notes.
Brackets
You can use tuplets to create brackets over notes for things like ligatures, by creating a tuplet that is “3 things in the time of 3 things”.
And of course, Ossias for variants or editorial suggestions are an absolute doddle! If you’ve got any other tips or wishes, add them here.
One wish would be footnotes that display at the bottom of the page, but linked to a note or measure. Possibly a kind of System Text that displays below the entire system might be good enough.
Thanks - this is really useful.
Re the square brackets round dynamics: the prefix/suffix method works up to a point, certainly, but the square brackets should not be italic, and consequently there appear to be considerable gaps either side of the dynamic marking. Editorial brackets really do need to be square; the round ones are not suitable for that purpose, though obviously they have a place as normal cautionaries. (I’ve been hoping for decent editorial square brackets since Sibelius days; I’ll keep hoping!)
That’s a great summary of how we can currently make editorial markings in Dorico — thanks for posting it.
To add to this list of Editorial techniques, I’m really enjoying how easy it is to use the Lyric Chorus font for displaying editorially ‘filled-in’ lyrics in Italics, where the source material has ‘ditto’ marks (or nothing at all).
… and that still leaves me with the Translation Font for figured bass! 
