[XeTeX] Is it possible to use letters from main font for unicode-math?

[Vasile Gaburici]

The reason I'm asking this is that I'm investigating a conversion of
MnSymbol to an OpenType MATH font. But MnSymbol doesn't have any
letters of its own, it relies exclusively on Minion Pro for those. So,
we'd need a hack in XeTeX to accommodate this. Granted, the font
converted this way won't work in Office 2007 because it requires the
letters to be present in the Math font itself. But I don't really care
about Office...