very cool. I had the same idea for clover a while back to have separate databases per thread, but i gave up, because clover has no merging functionality (at least I did not understand how to use it). Also the setup of clover in lucene was to have a one-for-all clover database...
I checked you patch, very minimal invasive, cool! Just a few test policy files and the "hack" with relative paths for the database. I just wonder why the "clover" target is sometimes included as dependency to the new tasks, but maybe thats just copypaste.
A further imprvement could be to have a root-level report with lucene+solr for Jenkins. Should be easy. Maybe make the report generator a macro in common-build.
I still like the clover reports more (they have much nicer report output and functionality), but jacoco is fast and - as Grant said - does not need to istrument classes at compile time. So you can quickly switch between jacoco and non-jacoco without recompiling (for Clover you had to "ant clean" first).
+1 to start with this. Maybe we can remove the invasive clover at some time - but not under the scope of that issue!