how close are we to having an "addAt" or "insertAt" feature?

[Kieren MacMillan]

Hello all,

What would be involved in developing a feature to add notes or tweaks at an 
arbitrary moment within a music expression?

e.g.

    global = \repeat unfold 100 s1
    music = \addAt (4 3/8) \global \once \override RehearsalMark.extra-offset 
#’(-1 . 0)

where (4 3/8) means “in the fourth measure, at the moment of the 3rd eighth 
note”

This would allow “perfect” separation of content from presentation — a very 
welcome thing to my mind.

Thanks,
Kieren.