Ok, this is an ugly hack, but if nothing else works, well, this will work. This is from The PracTEX Journal, 2006, No. 4, Article revision 2006/11/03, Hypertext capabilities with pdfTEX
by Federico Garcia.
\pdfannot width w height h depth d { /Subtype /Text
/Contents (htexti)}
This is via pdfTEX primitives, so I have no idea what is going on with it. It makes a postit note in the document that you click on to see the text.
Instructions:
The three dimensions w, h, and d are all LATEX dimensions. But the one
that is important is h (the height), because it determines where the
note appears in relation to the text baseline. It is also a good idea
use a \qquad after it.
Also note: This works fine in Adobe. It doesn't work at all in Sumatra PDF. Well, the note shows up, but you can't click it. So this is a bit risky (So is javascript for that matter, as a lot of PDF readers avoid it for security reasons).
A bit more from the article:
One thing to have in mind with post-it notes is that their exact
behavior (color, size, when it opens, how it closes, etc.) is not very
much standardized, and tends to change from version to version of
Acrobat Reader. That said, the note can be of a different color if,
between the brackets of \pdfannot, /C [r g b] is appended; a title for
the note is determined with /T (title), and the note can show up open
by default with /Open true.
Well, that is one way. A terrible, terrible way, but it would technically work.