2.19 tempo mark question

[Shane Brandes]

O.k. here is the desired result  "Rubato. quarter note glyph = c.90."
there used to be a way to hide the resulting parenthesis and also
adding a little bit of text between the equal sign and the numbers.
e.g. \set Score.tempoEquationText = "= ca." so I tried \set
Score.MetronomeMark.EquationText = "= ca." but to no effect. I checked
the back ends for MetronomeMark and equation text isn't there. Nor is
a hideparanthesis I suppose I could do it through markup, but the old
manner seemed more useful somehow. So I am I missing something
obvious?

Shane