[XeTeX] using a different small caps font

[Dominic Yu]

If a font doesn't have a bold version, I know I can specify one
easily, like this:

\setmainfont[BoldFont={Times Bold}]{Doulos SIL}

But I'm afraid I can't figure out how to do the same with small caps.
This doesn't work:

\setmainfont[SmallCapsFont={Hoefler Text}]{Doulos SIL}

because the small caps is embedded inside the font, and I don't know
how to invoke the font with, say, the Letters=SmallCaps feature
already on. Please help!