[XeTeX] Determining current font

[James Crippen]

This is a XeLaTeX/fontspec problem, I've got a current SVN checkout of
XeTeX and fontspec 1.17, both on Mac OS X 10.5.3. I'm trying to write
a macro which selects a specific nonstandard symbol from two different
fonts. Specifically, both Charis SIL and Adobe Minion offer a slashed
zero symbol, but encoded differently for both. In addition, both have
different OpenType features used to access them (Charis SIL uses a
variation of the empty set, and Minion uses the SlashedZero feature,
IIRC).

My idea was to simply check which font was currently in use when the
macro is called, and specify one or the other ¨Àlyph dependin¨À on the
font. But I've been looking around in the documentation and sources
for fontspec and XeTeX and I can't find a macro that returns the value
of the current font which I could use for a conditional.

Is there a way to get information about the current font in use? A
macro or variable I can poke at?

Thanks,
James Crippen