Ooops. Sorry, it was too small on my laptop screen for me to really see the symbols.
FYI, I just discovered two days ago that you can now actually display icons in the buttons, either by referencing their path, or, in case they are part of the "standard" internal icon sets, by referencing their "#name" (Control [Directory Opus Manual]).
If you add this property and also provide a title to your button + set the very last property in the button UI editor ("etiquette" in french) to yes, you get both the icon and the title displayed on your button.
I also used unicode to add meaning to my buttons, but I think I'm gonna use this more now.
I didn't catch when this was added, but I don't have this on my 13.1 install (but it's on 13.7.8).
Only drawback I see : I think this does not get shared in the ".opusinstall" ... but I did not test.
Found it here : Directory Opus 13.6.2 (Beta)