[jackie_w]

You don't need a calibre plugin to make your epubs access a font sideloaded into the Kobo's /fonts directory - instead of embedded into the actual epub. All that should be required is

• Rename the Stix Maths font to Kobo standards, probably STIX Math-Regular.otf (case sensitive)
  and copy it to /fonts on the Kobo - followed by a reboot as it's a new font.
• a slight change to the @font-face css, namely
  

  @font-face
      {
      font-family: "StixRegular";
      src: url("res:///fonts/normal/STIX Math")
      }
  .maths
      {
      font-family: "StixRegular";
      }

The red names MUST match exactly the internal font name of the Stix font file. The blue names can be anything (alphanumeric I think) as long as they match each other.

Again FYI, if you're a calibre user, you can have the above css automatically added to epubs during the calibre/Kobo send-to-device process, leaving the calibre master epub unchanged.

However, I think the OP wanted to embed the font so that it would be sure to work on any epub reading device/app not just the Kobo.