Problem with custom font feature for addition kerning in OpenType font

I try to add some custom kerning to an OpenType font used with fontspec under lualatex. The real-world example uses the Brill fonts, but as they are not part of TeX Live I use Libertinus in the example below.

I want to add some kerning between specific characters with diacritics and superscript numbers. The only combination which works is the direct usage of the unicode superscript numbers. As soon as I use textsuperscript, which is redefined by realscripts to use the correct figures, the extra kerning is not applied.

Is there any way to achieve what I want while using textsuperscript?

% !TeX TS-program = lualatex
documentclass{article}

usepackage{fontspec}
% Values below are for demonstration purposes only
directlua {
   fonts.handlers.otf.addfeature {
      name = "supkern",
      type = "kern",
      data = {
         ["ī"] = {
            ["¹"] = 500,%
            ["two.sups"] = 500,%
         },
         ["š"] = {%
            ["one.sups"] = 500,%
            ["two.sups"] = 500,%
         },
      },
   }
}
usepackage[defaultfeatures={RawFeature={+supkern}}]{libertinus} %Brill in real life
usepackage{realscripts}

begin{document}
   ītextsuperscript{12}
   
   ī¹
   
   ī{addfontfeatures{VerticalPosition=Superior}2}
   
   {addfontfeatures{RawFeature={+supkern}}īaddfontfeatures{VerticalPosition=Superior}12}
   
   štextsuperscript{2}
   
end{document}

documentclass{article}

usepackage{fontspec}
% Values below are for demonstration purposes only
directlua {
   fonts.handlers.otf.addfeature {
      name = "supkern",
      type = "kern",
      data = {
         ["ī"] = {
            ["¹"] = 500,%
            ["two.sups"] = 500,%
         },
         ["š"] = {%
            ["one.sups"] = 500,%
            ["two.sups"] = 500,%
         },
      },
   }
}
usepackage[defaultfeatures={RawFeature={+sups,+supkern}}]{libertinus} %Brill in real life
RenewDocumentCommandtextsuperscript{m}{#1}
begin{document}
   ītextsuperscript{12}

ī¹ 
end{document}