With `balloonGrobText' one can add a balloon for grob. But this does appearently not work for clefs.
Example:
\version "2.20.0"
\new Voice \with { \consists Balloon_engraver }{
\override Voice.BalloonTextItem.annotation-balloon = ##f
g'4
\balloonGrobText #'Flag #'(1 . -2) \markup{"Flag"}
\balloonGrobText #'Clef #'(1 . 2) \markup{"Clef"}
\clef F
b8
}
How to add the balloon-text for the F-clef?