If so, I’d like to know where to find the reference in the Rules of UEB 2013 and, if not, please advise as to the way it should be handled. These are not parts of equations.
What you have is actually the underscore (or low line) followed by a hyphen. Because there is no ambiguity between these symbols you braille them just as they are in print…the underscore (dots 46, 36) followed by the hyphen (dots 36).