[XeTeX] Scaling a font

[Tim Eyre ティム]

Is there any way to tell XeTeX to scale a font from within the font
name rather than with the "at 10pt" part of the spec? I'd like to do
this as part of creating fake smallcaps.

What I have in mind is something like:

\font\sc="MyriadPro-It:+onum,scale=800" at 10pt

where the scale=800 means that the "at 10pt" actually yields a font
that looks the same as one that had been specified as

\font\sc="MyriadPro-It:+onum,scale=800" at 8pt

Any help greatly appreciated,

Tim