I’m looking for something that I can place in-between notes, which will cause them to move out of its way. A ‘spacer’, you might call it.
Might there be such a thing?
As usual, I used a Jazz ornament. 
I wanted to add a ‘custos’ to the end of each line, and needed an easy way of shifting the notes out of the way to accommodate it.
Just curious. It looks as though you used glyph #EA02 in the Bravura font. If you entered it as an ornament, what did you attach it to and how did you get Dorico to space for it?
I replaced the Jazz Doit glyph with EA02, then added it to the note. Then in Engrave mode, select the Doit by itself, and Command Alt Right a few times to move it, pushing everything else on the line back.
Then move it vertically to the correct pitch.
Obviously, if the line break moves, then it’s in the wrong place, but luckily I’ve got System Breaks everywhere, as I’m preserving the source lines!
Thanks, Ben. I’d forgotten that Dorico inserts space after the note for the doit. Also wondering: I just happened to find EA02 under Medieval and Renaissance Miscellany. There are so many glyph ranges, how is one supposed to find a particular glyph and enter it?
Mostly just by browsing. The ranges aren’t very helpful, particularly when in alphabetical order – Common Ornaments under C, and Standard Accidentals under S. (Or is it the other way round…?)
Pff!! There must be a better way to select particular glyphs. I tried entering EA02 as text (using PopChar) but it turned up as a generic, unrecognised symbol.
Sounds like you weren’t using Bravura Text, but just whatever the default text font was.
Sure, but as soon as you select Text instead of Glyph, the ability to choose the font disappears.
Is this as a Playing Technique, or a Text Object, or in the Music Symbol Editor?
I was trying to change it in the Music Symbol Editor.
Just select the glyph from the Mediaeval & Renaissance Miscellany range on the right hand side, and Add it.
I know, that’s what I did but, as I wrote, I just happened to find that particular glyph in the myriad of ranges. Wouldn’t it be easier if we could just select a glyph without having to search for it in a daunting selection of seemingly random range names?
I just ran across this same case yesterday. I was doing this hybrid motet/anglican chant thing and wanted custos as well, particularly for the tenors who shift from open score 8vb to condensed with basses in the bass clef (necessary for formatting reasons to get it all onto one page). I ended up adding a single measure of 1/4 at the end of the lines where I wanted custæ(?) custosi? lol. I then added a quarter note to indicate the next pitch. I hid the time signature and then went into the notehead sets and created a new set where the quarternote is the symbol indicated above. Then back in the score I applied that notehead set to the affected measures. The effect is pretty good, I think, and this way the custos are actually related to pitches (although they would mess up playback.)
Edit: I think costos should be brought back into favor, not just for historical editions. They are marvelously helpful and I’m sorry they ever fell out of use. I even mark pitches for myself at the bottom of pages in my organ editions from time to time. But for singing and page turns or condensing changes, they are very helpful.
Use the “Suppress Playback” switch in the Properties panel to stop them playing. That’s a nice method, though.
pl. custodes 
In cello parts I play I use them quite often, on page turns, tricky position shifts at the end of a line, clef changes…
Learn new things every day! 