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Hrant H. Papazian said:
Axes that are actually just toggles will be needed.
I have been thinking for some time that this sort of facility would have been a smart addition to the OT Font Variations specification: Basically the ability to point a GSUB rule to a position within the interpolation matrix rather than just a separate glyph outline. For things like small caps and superiors/inferiors, as Jeremy and Adam point out.
Perhaps in version 2.0.