I mentioned that properties that got indexed due to an aggregation are not considered for excerpts (highlighting) as they are not indexed as stored fields.
See the attached patch that implements a test for excerpts in LuceneIndexAggregationTest2.
It creates the following structure:
where both strings (the bar property at foo and the bar property at jcr:content) contain different text.
Afterwards it queries for 2 terms ("tinc*" and "aliq*") that either exist in /content/foo/bar or /content/foo/jcr:content/bar but not in both. For the former one the excerpt is properly provided for the later one it isn't.