I've found the following 3 styles in use:

1. \indextext on the line before the \item (example)
2. \indextext on the same line as the \item (example)
3. \indextext on the line after the \item (example)

One thing style 3 has going for it is that it is consistent with the existing style rule for index entries for paragraphs (which are to be placed on separate lines after the \pnum).

I'd be happy to submit a patch once a style is chosen. I guess the style guidelines could be updated to show an index entry for a list item, too.