[XeTeX] changing font for numbers in math mode?

[Ernest Adrogué]

Hi there,

I was doing some tests with various math font packages
such as fourier, mathdesign, txfonts, etc., and I realised
that these packages could also be used with other text fonts
other than the fonts they were intended to be used with.

For example, mathdesign uses math symbols (italics and greek
letters) from Charter, which wouldn't look bad with Charis SIL,
given that Charis SIL itself is derived from Charter. The 
problem is that if you try to use Charis SIL with 
mathdesign/charter, the numbers in math mode are from Charter
and do not match the numbers that are not typeset in math mode,
the result being totally horrible. 

So I was wondering if it is possible to change the font only
for numbers in math mode, while leaving everything else
unchanged. The command \setmathrm doesn't seem to do anything.

Cheers,
-- 
Ernest