[Bill O]

On iOS, MSPro seems to ignore typeface* in font directives (everything seems to be rendered in a default serif typeface) but honors size. Silver lining: I can just leave all the font directives in place for use by e.g., txtmus or EasyABC. 

*in addition to my usual %%gchordfont Avenir-Book, I've tried Sans, sans, sans-serif, san-serif and Arial. Perhaps I'm missing something about the implementation.