In triple-triple meters (e.g. 9/8) it’s common for a note to be nine of a smaller note. Early-music editors (e.g. Schrade) have adopted a notational convention for this: a double-dotted note, but with one dot above the other rather than side by side. So a double-dotted breve is nine minims, a double-dotted semibreve is nine semiminims, etc. How can I do that in Dorico?
I don’t think it’s possible, at least not natively inside Dorico. You can create a playing technique (or a text item), and manually place it to the right of the long note. In order to have the long note last 9 eighths/quavers, you could place it inside a 8:9 or 4:9 tuplet, depending on the notehead you prefer.
Maybe a bit like this?
Thanks; I can work with that. It would be nice if I could associate a playing technique glyph with the rule “always make this note a hidden 4:9 tuplet”, but I don’t think Dorico does that yet. Let’s call that a feature request 
It’s a pain that playing techniques can only appear above or below the note: for each such double-dotted note, I have to go into Engrave mode and move it manually to just after the note. And hope that they’re all similarly positioned. Fortunately, this piece doesn’t have a lot of nine-beat notes.
Oh, and “hiding” a tuplet is anything but obvious: you have to select it, bring up the appropriate properties window, turn on “number” and “bracket”, and somehow the numbers and the bracket both disappear. There’s nothing discoverable like, say, “hide tuplets”.