Harfbuzz is capable to deal with the feature tags in OpenType. it would be a good idea to support it in fontconfig as the configuration.
the proposed fix:
I don't know if we should check with Harfbuzz or own code like we did in capabilities if the given feature tags are valid or not.
updated the naming according to the discussion at the list.
I don't think there is much configuration value in looking deeper in the font. *Unless* we want to add things like being able to query whether the font has a feature, etc. But then, the only think I can think of that can use something like that would something vaguely like requesting a small-caps font. Even then it's not clear what a config would look like.
I think we should go ahead and commit this and worry about more complex things when a concrete need / proposal comes up.
I'll switch Pango over when you commit this.