Hmm, I just tried this unsuccessfully as well. I can copy glyphs from smufl.org, but I can’t copy them from GitHub. And the smufl site doesn’t have the 12/8 glyph. I tried copying 1, 2, but they were just superimposed on each other in shift-x text - I couldn’t get the cursor to move forward.
I was able to use the Opus font to create a reasonable facsimile of 12/8, if that helps.