enharmonic problem with \transpose - should we modify it?

[Janek Warchoł]

Summary: { \key c \minor  \transpose gis as { es } } produces feses in
output. I think it would be better if it outputted es.

I have a piece written in es minor, which contains a lot of sharps and
naturals (along with passages of flat notes). I'd like to "transpose it
enharmonically" so that sharp notes become flat notes a scale step above,
natural notes become double-flatted notes, but regular passages of flat
notes aren't affected. \transpose gis as works for this with one exception:
it produces ceses and feses everywhere instead of bes and es (which would
fit the key signature better). This disturbs the harmonic structure: when a
passage like this { \key es \minor as' bes' des'' bes' ges' es' as' as' } is
transposed, most flat notes aren't affected (because \transpose doesn't
create triple flats), but some are - and intervals change, for example first
one changes from major second to diminished third.
I tried using \naturalizeMusic from NR 1.1.2 on it, but this results in
problems in other places (because \naturalizeMusic ignores key signature).
I think it would be good to either modify \transpose function, or add a
function which is similar to \transpose, so that described results can be
acheived.
What do you think?
I searched for transpose in sources and checked all results that were not
docs and regtests, but my scheme reading skills are too low: i cannot
identify where it is defined (i have an impression that it's definition
isn't all in one place). Please give me some pointers.

cheers,
Janek