Going back to the properties approach, did you try adding a keep-together.within-line="always" property to the superscript.properties attribute-set? I didn't test it, but that should work to prevent hyphenation, or any line break at all in the superscript. If your intention is to prevent hyphenation but allow line breaks between words in a superscript, then my suggestion would not help.
Bob Stayton
Sagehill Enterprises
bobs@sagehill.net