I use this for educational exercises sometimes so I’ve added a 0-line instrument to Dorico with a bit of hackery. Be aware this is not supported, and will get overwritten with any update so be sure to back up both your original files before beginning, and then your edited files once you have them the way you want. Here’s a walkthrough on how to add a 0-line (or really any instrument) to D4 in Windows as below:
In Program Files\Steinberg\Dorico4 open your instruments.xml file in a text editor. I use Sublime Text but there are obviously plenty of alternatives. I simply copied the InstrumentEntityDefinition for a Piano and pasted it at the end of the file to give me something to start with. Change the name, give it unique entityID and nameID entries, change numStaves to 1, change numStaveLines to 0, and I also changed the clef ID to clef.invisible. It should look something like this:
Next, open your instrumentFamiliesDefinitions.xml file. I added the instrument entityID to the Sketch category, but you can add it wherever you want. It should look similar to this:
Next, in the l10n folder open the instrumentnames_en.xml file. (If you’re using English obviously.) Copy and paste an existing entry, then change the nameID, then change the staff names to whatever you want. I sometimes use this for a Cue staff, so I guess I have it default to “Cue” but this is obviously easy to change within the program.
Restart Dorico and you should now have a 0-line staff instrument! You can easily create a 1-line staff instrument in a similar manner.