I suspect what I'm looking for ultimately is what other values are possible for the IndexEntryType other than the IndexPageEntry value that is used when it is a page reference entry.
Err. I don't have time to look at this right this second, but try exporting some hyperlinks to IDTT and looking at how they are formatted and see if you can extrapolate from that to cross-reference. I believe they are fairly similar.
It is possible that it might not be possible though.
I have been exporting various things to IDTT to see what the result would be, with nothing really helpful as a result. I'll try hyperlinks, but reading of the Adobe guide to InDesign Tagged Text and also just looking at the InDesign scripting object model leads me to believe that there must be specific tags to create index-specific cross-reference tags.