I don't think you'll ever be able to get text display as superscript in the TOC. Not because of Sigil, but because I doubt you can express a superscript in the NCX file. The NCX doctype is, well, "ncx". It's not HTML. I doubt there's a "<sup>" tag (I could be wrong).
What I would do is this:
Code:
<h3 title="TITLE^24">TITLE<sup><a href="#sdfootnote24sym" id="sdfootnote24anc">24</a></sup></h3>
That is, display the superscript correctly in the book, but use the common "^" notation for the TOC. The "title" attribute overrides the content of the element for the TOC generation.