[XeTeX] Defining small caps font from SIL Charis, Doulos

[ahoenig at jjay.cuny.edu]

Hope this is a quick query.  I'd like to define small caps fonts for use with plain XeTeX 
using the SIL Charis and Doulos fonts, both 
of which appear to contain small caps (at least when I look at the full font repertoires 
using Font Book on Mac OS X).  Using +smcp 
or `Letter Case=Small Caps' doesn't appear to work.  THanks for any info!

---Alan