Teach fast FastVectorHighlighter to highlight "child fields" with parent fields

I've been messing around with the FastVectorHighlighter and it looks like I can teach it to highlight matches on "child fields". Like this query:
foo:scissors foo_exact:running
would highlight foo like this:
<em>running</em> with <em>scissors</em>

Where foo is stored WITH_POSITIONS_OFFSETS and foo_plain is an unstored copy of foo a different analyzer and its own WITH_POSITIONS_OFFSETS.

This would make queries that perform weighted matches against different analyzers much more convenient to highlight.

I have working code and test cases but they are hacked into Elasticsearch. I'd love to Lucene-ify if you'll take them.