[XeTeX] Tracking for small caps?

[Wolfgang Keller]

Hello,

with Microtype, it's easy to get automatic tracking for all small caps
text within the entire document by putting 

%\DeclareMicrotypeSet*[tracking]{my}{ font = */*/*/sc/* }
\SetTracking {encoding = *,shape = sc }{foo}

into the preamble. What would be the corresponding preamble line(s) for
XeTeX?

Please note that the point is to _avoid_ having to add

\addfontfeature{LetterSpace=foo}

to every piece of small caps text all over the entire document. And
also to _avoid_ using a seperate font instance for small caps text.

TIA,

Sincerely,

Wolfgang