If I understand you correctly, you want to add a new articulation script
(rather than patching the code for dynamic marks). That's quite easy, if
you are a little bit familiar with metafont. For that purpose, you
should
* add the articulation sign to the feta font; see file
mf/feta-schrift.mf;
I guess the direction (up/down) does not matter for espressivo marks,
hence a single character "espressivo" should suffice; you may want to
have a look at the sforzatoaccent as an example of a similar character
that is already in the font;
* tell lily about your articulation sign by adding it to the file
scm/script.scm (twice the same character "espressivo" for both
directions, up and down);
* add a handy shortcut to the file ly/script-init.ly
* for testing, add it to input/test/script-chart.ly
* for documentation, add it to the Articulations section (currently
section 5.7.8) of the user manual; see
Documentation/user/notation.itely.
And, whenever changing feta-schrift, don't forget to clean the font such
that all font files will be rebuilt; otherwise, you will not see any
change, or the font will be messed up (as inserting a new character
changes the numbers in the font tables).
lily-041126.patch