I came across some old documentation that mentioned using the exact="yes" attribute in conjunction with <idx:key>. I tried it in a test file and found that adding it does in fact carry over as a change in the dictionary created using kindlegen.
Unfortunately, it does not appear that this will be useful in practice. Exact seems to do nothing when used with <idx:iform> and the current Kindle publishing guidelines state that <idx:key> is deprecated and should not be used.
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