[XeTeX] XETEX cannot access OpenType features in PUA?

[Aleksandr Andreev]

mskala writes:

>> Can what you're trying to do be implemented by zero-width negative-bearing
glyphs instead?

Not in any straightforward way. The problem is that the vertical
position of the red marks is dependent on the base neume. So with a
negative-bearing glyph, you have to do some kind of contextual
substitution, which then creates the same problem in terms of
rendering color.

I'm trying to experiment with LuaTex and ConTeXT right now and see
what happens there.

Looks like the problem of typesetting text simultaneously in black and
red is as insurmountable today as it was in 17th century Muscovy.

A