After making the change shown above in solrconfig.xml, a commit is enough to make the number of open file handles grow:
curl http://localhost:8983/solr/update --data-binary '<commit/>' -H 'Content-type:text/xml; charset=utf-8'
lsof shows that many of these file handles point to files in data/index, which have been deleted during the commit:
lsof -p 9563 | grep data/index | wc -l (where 9563 is my solr's process ID)
shows 398 file handles after a few commits, although my data/index dir contains only 47 files.
So it looks like something is keeping useless open handles to "old" index files after a commit.