stroke hand finger instructions in "ghost voice"

[sylvius]

Hi all,

I am trying to separate musical content (especially pitch and rhythm) from 
other pieces of information (such as fingering instructions).  I’m aware of the 
existence of the edition-engraver and Gilles Thibault’s  \addFingering function 
(http://lsr.di.unimi.it/LSR/Item?id=768) but I actually prefer to use separate 
“ghost voices” (voices containing spacer rests).

I discovered that it is possible to attach fretting hand information to spacer 
rests and align their horizontal and vertical position as follows:

%%%%
\version "2.21.0"

fretHand = { s1-\tweak X-offset #-1.5 -\tweak Y-offset #1 -0 }
music = { e''1 }

\score {
  \new Staff <<
  \new Voice { \music }
  \new Voice { \fretHand }
  >>
}
%%%%

On the other hand it doesn’t seem possible to attach and align "right hand 
finger” information to spacer rests?!

%%%%
\version "2.21.0"
music = { e''4 e'' e'' e'' }
#(define BH rightHandFinger)
bridgeHand = { 
  s4-\BH #4 
  %s4-\finger "a" 
  %s4^\markup "a" 
  %s4-\tweak X-offset #0 -\tweak Y-offset #3 ^"a"
}
\score {
  \new Staff <<
  \new Voice { \music }
  \new Voice { \bridgeHand }
  >>
}
%%%%

But maybe there are better or more elegant solutions to this problem?  Thanks 
for any hint!

Sylvius