Thanks for waiting while I researched this interesting question. First of all, the β isn't a modified letter, it's a non-Latin letter in the German alphabet. World Braille Usage shows dots 2346, but then the other foreign language symbols for the umlauted letters would also have to be used instead of UEB modifiers, and would necessitate the use of non-UEB code switch indicators ... way too complicated for a novel. There is no provision yet in the UEB code for that letter. The International Council of English Braille is planning to establish a symbol in the future. In the meantime, the easiest solution is to substitute "ss" for the β and explain this in a transcriber's note.