[Wendy Cutler]

For now I use special song files as separators. ...

Right, thanks. That's what Goeff Bacon suggested. I am doing the same thing for separators, using the Add Placeholder function (instead of an external program to create the file):

Add Placeholder – Creates a new song with a single blank page in it. This can be used to create and prepare a song that will later contain a real file instead of blank pages

Looking at the set list, you click the three vertical dots at the upper right, select Add Placeholder. 
It asks for Position and Name. You can select the emoji keyboard at that point to include coloured pictures in the name. Or you get the same editing functions as for any regular file. In contents edit, you can select the A to add text to the file, or in the file description edit, you can edit the file name. If you don't see emojis when you edit the title, click the three vertical dots on the upper right of the keyboard.

I am attaching my set list now with a set separator. Since I used that musical notes emoji as the first character, when I edit any set list, the list of tunes to select from has all the ones with the musical notes emoji at the top, so adding a placeholder adds it to your available files for any set list. And you can drag them to where you want them like you can for all the songs in a set list.
   

That's my workaround. But I don't just want it for separators - I want to highlight tunes in a set list for some reason - either to read them at the gig, when I'm mostly not reading anything, or to indicate that they need practice. So in that case, it's not a matter of a separate file. For now, I will add an emoji to the end of the filename to be that indicator.