In Yannis Haralambous'
Fonts and Encodings, he gives an example of an OpenType single adjustment positioning:

It's pedagogically a very neat example of how GPOS and OT features work: you put a simple positioning rule inside a feature (
smcp
); turn on the feature and the parentheses drop down vertically.
But has this (or anything like it) been seen in fonts in the wild? Does it even make typographic sense?
Comments
*some even sub in alternate designs fit for each alphabet
The classic example is the underlining in InDesign.
Granted, underlining in InDesign sounds so faux pas. But I do know some publications that use this feature to create a text highlight style in print, so it’s not as unusual as some might think.
In this demo, my font is testing the possible use of GPOS to reposition the Catalan middot (instead of having a separate periodcentered.CAT glyph and using substitution; see also this discussion.). You can see what happens:
The same would happen for {smcp} punctuation in the case you cite. Unfortunately.
The GPOS approach is technically sound. But the implementation in the wild may not be.
I don't believe I ever tested Web environments. Perhaps they are more accommodating.
Pity this doesn't seem to work with Apple's typography panel. I can select «Lower Case > Small capitals» and «Upper Case > Small capitals», but the punctuation does not seem to be affected. But that's just one of many things that don't work as advertised with that typography panel (at least in Keynote).