Hi again,

The OpenType digit display featues with lookup tables 'tnum', 'lnum', 'onum', 'pnum' would be useful for people doing careful typography.

The slashed zero can greatly improve readability in some cases.

I think they should be pretty easy to implement.

Each should execute a simple substitution of a digit with glyph specified in the font's lookup.  They should be accessible from any script.
These should all be turned off by default, and turned on at a user's discretion.
The only complication seems to be that some of them override others.

For details: http://www.microsoft.com/typography/otspec/features_pt.htm