I want to enter some text using a music font. i.e. Bb with the b being a music font.
Have you tried using the Bravura Text font?
Though note that it’s not as simple as just switching to Bravura Text and then typing e.g. b for a flat sign. Bravura Text uses the SMuFL font mapping, which means that none of the musical symbols can be typed directly from the keyboard. You will need to copy and paste the symbols from extended Unicode positions, e.g. using this site.