simultaneous rehersal marks, tempo indication, and text marks

[Graham Percival]

At the moment, we can't have text and rehearsal marks at the same
place, i.e.

\relative c' {
  c1
  \mark \default
  \mark "play violently"
  d
}

This can be faked by using a \tempo mark:

\relative c' {
  c1
  \mark \default
  \tempo "play violently"
  d
}

But if you want to display an actual tempo marking at the same
place, you need to resort to silly hacks like grace notes,
invisible notes, or defining a text mark involving both the
rehearsal letter and the actual text you want to show.

What would be involved in making a clean solution for this?  I
imagine that a separate TextMark engraver (just like the
RehearsalMark engraver and MetronomeMark engravers) would do the
trick, but that's a bunch of icky C++ code.  Is there any way to
use scheme to create a new engraver that behaves like an existing
engraver (i.e. TextMark), but has its own data (so it doesn't
merge the rehearsal mark event with the "text mark" event?)

- Graham