Aligning note to different voice across staves

[Thibaut Horel]

When trying to typeset the attached bit of music (this is bar 18 of the 
Fugue in BWV 881), I encountered a problem captured by the following 
minimal example:

------
\version "2.22.0"
\new PianoStaff <<
   \new Staff << \relative{e'16 f g e} \\ d'4 >>
   \new Staff { \clef bass e8[ g] }
 >>
------

In this example, I would like the e8 in the lower staff to be aligned 
with the d4 in the upper staff. However, because of a note head 
collision, the d4 in the upper staff is shifted to the right, and the e8 
from the lower staff is instead aligned with the e16 in the upper staff.

I was able to obtain the correct behavior by adding \once\override 
NoteColumn.X-offset = 1.25 before the e8 in the lower staff. But there 
are two things I don't like about this solution:

1. if I understand correctly, X-offset is measured in staff-space, so I 
am worried this might not scale properly when changing the note head 
size (for example by using a different font). It would be nicer to be 
able to express the offset in note head width, as for the force-hshift 
property for example.

2. My solution requires trial and error to find the correct value for 
the X-offset, whereas what I am trying to achieve can be expressed 
easily at the semantic level: I would like the e8 in the lower staff to 
be considered part of the second voice of the upper staff for vertical 
alignment purposes (in particular it should be shifted at the same time 
in case of a collision).

Is there a better way to achieve what I am trying to do? Any insight 
into this problem would be greatly appreciated.

Thanks,
Thibaut

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