In some way it would be desirable for some cases to have ledger lines at
Voice level, e.g. in
<< a'' \\ c' >>
But it becomes unwieldy soon; what about
<< a \\ f >> ?
The only sensible way to deal with this would be creating an association
between a NoteHead and the particular LedgerLine on which it sits; then
again, a case like
<< h \\ e \\ g >>
(always assuming treble clef) would not allow for that either.
Generally, ledger lines are to be thought of as an extension to
StaffSymbol – which clearly shows that they need to live in Staff.