[XeTeX] Script specific digits

[Musa Furber]

Dear XeTeX list,

The glyphs for the Arabic digits used in Latin languages occupy the 
range between 0x0030 and 0x0039.

The glyphs for the Hindi digits used in Arabic occupy the ranges 
between 0x0660 and 0x0669. Additionally, the are additional Indic 
digits between 0x06f0 and 0x06f9.

Not all fonts have a full set of glyphs for the various types of 
digits. Al Bayan, for example, doesn't have the glyphs for 0x0030 
through 0x0039.

Does anyone know an appropriate way to map one range of digits to 
another, e.g., have 0x0030 through 0x0039 map to 0x0660 through 0x0669, 
or the reverse?

I started adapting Boris Lavva's Hebrew package for Babel to work for 
Arabic under XeLaTeX. It turns out his package was easier to adapt to 
XeLaTeX than ArabTeX.

This numbers thing has me stumped.

Regards,
Musa