[XeTeX] XETEX cannot access OpenType features in PUA?

[Aleksandr Andreev]

Hello folks,

I'm working on an implementation of a font for Znamenny Neumatic
Notation in Unicode (see here:
http://en.wikipedia.org/wiki/Znamenny_chant)

Without getting too much into the details, I'll just state that the
notation includes base characters (neumes) and diacritics (red marks
or black marks). Sometimes, the neume may also change its shape, and
this is controlled via control characters in the proposed standard.

As the standard has yet to be proposed to Unicode, in the font the
glyphs are currently mapped to the Private Use Area. For the
implementation, the font makes heavy use of OpenType technology. In
particular, anchor points define the positions of red marks and black
marks over the neumes. The functions of control characters are handled
via a Ligature Substitution.

Now I'm writing up documentation for this standard in XeTeX.

But, it appears that XeTeX is unable to handle these features. For
example, consider this snippet:

\documentclass[12pt,a4paper]{article}
\usepackage{color}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage{fontspec}
\defaultfontfeatures{Mapping=tex-text}

\newfontfamily{\moo}{MezenetsUnicode}

\begin{document}

{\moo \color{red}{}}

\end{document}

The control character  is supposed to modify the neume   (sort of
like a variation selector). Then, the diacritic  is supposed to be
placed in accordance to the appropriate anchor point defined in the
font.

(None of these symbols will mean anything to you as you don't have the
MezenetsUnicode font that I've designed. You can download it here if
you wish to experiment:  http://www.ponomar.net/files/mezen_uni.ttf

Please do not distribute this font -- it is only pre-beta software.)

Instead, I see the neume, a box for the control character, and
diacritic floating over nothing.

The font is OK, as the same characters in Pango and FIrefox produce
the desired results, so there is something wrong with the way XeTeX
handles GSUB and GPOS in the PUA. Is this a bug?

I'm running xelatex version 3.1415926-2.2-0.9995.2 on Ubuntu 11.10.

Thanks for any insights,

Aleks