I've done a very dirty test which showed that a properly set glyph
bounding box solves this perfectly (see in the attachment how the
bounding box should change).
However, there is a problem. Currently bounding box is set by
procedure draw_outside_ellipse (defined in mf/feta-params.mf) - it is
not set in the definition of the glyph. How to set glyph bounding box
to something different than outside_ellipse bounding box?
You could define it in the "final" definitions of glyphs. Here is an example. Of course, these values have to be define in a clean metafont way.
fet_beginchar ("Brevis notehead", "sM1");
draw_brevis (1, 1);
draw_staff (-2, 2, 0);
set_char_box (.1 staff_space#, 2.1 staff_space#, .5 staff_space#, .5 staff_space#);
fet_endchar;
Bertrand