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After reading Tal Leming's fraction fever I decided to give this a go. I've long wanted to script a decent fraction feature, but couldn't get it right. This is working perfectly (for now ... I've only tested it in Indesign CS2 on Tiger).
Still in progress ... but Museo Sans will end up with nominators, denominators, superiors, inferiors, oldstyle figures (proportional & tabular) and lining figures (proportional & tabular).