If/once this is done, there would probably need to be a way to report font features supported in fc-list and fc-query, and, longer term, UI support (e.g. in the gtk+ font chooser and maybe system preferences?)

I went ahead and did this in pango:

    <match target="font">
      <edit name="pangofontfeatures" mode="append">
        <string>smcp</string>
        <string>ss20</string>
      </edit>
    </match>

And sent mail upstream to see if it makes sense to put it in fontconfig itself.

Re reporting the capabilities, fontconfig already has OpenType scripts:

capability: "otlayout:DFLT otlayout:arab otlayout:armn otlayout:brai otlayout:cans otlayout:cher otlayout:cyrl otlayout:geor otlayout:grek otlayout:hani otlayout:hebr otlayout:kana otlayout:lao  otlayout:latn otlayout:math otlayout:nko  otlayout:ogam otlayout:runr otlayout:tfng otlayout:thai"(s)

which, admittedly, is not that useful.  I don't know what would be a nice format to report the features in a user-friendly way.

On the other hand, one can argue that there is not reason to put this in fontconfig at all.  HarfBuzz knows all this, so GUIs can access it.  And eventually we will have a hb-query that is specilized to do these kinds of reporting.  I'm leaning towards that approach.