Tuning TrillSpanners and slurs - enhancement request?

[Ian Hulin]

LilyPond version is 2.15.41.

Currently the recommended and documented way of tuning \trill
interactions with a slur or script is in NR 1.3.1 using the TextScript
and Script script-priority property:

\relative c''' {
  \once \override TextScript #'script-priority = #-100
  a2^\prall^\markup { \sharp }

  \once \override Script #'script-priority = #-100
  a2^\prall^\markup { \sharp }
}

However, if you use \startTrillSpanner the TrillSpanner object doesn't
have the property, nor does it have the TextScript avoid-slur property.

How should users specify that a TrillSpanner should be should be
engraved within a slur for a passage such as the one below?

 \relative c'' {
  \cadenzaOn
  bf'2~(\fermata
  \times 4/6 {bf16[ c bf g d g]}
  \times 4/6 {bf16[ c bf g d g]}
  \times 4/6 {bf16[ c bf g d g]}
  bf'2~\startTrillSpan bf4~\stopTrillSpan bf16[ a] bf[ f8.])
  \cadenzaOff
}