[XeTeX] character offsets in math fonts

[J P Blevins]

If I use the mathpi package with XeTeX, all of the characters in the
Mathematical Pi font sets are offset by one character. So $\alpha$ gives
'beta' (0062), not 'alpha' (0061) in Math Pi 1, $\mathbb{R}$ gives
'blackboard S' (0053), not 'blackboard R' (0052) in Math Pi 6, etc. The
same source file gives the expected characters in pdflatex.

Is there any simple way to correct this?

-Jim