There was talks about throttling gerrit, which is also running in jetty. The same basic mechanism could be reused, so extracting this code to a separate project / jar might help code reuse.
Note that the current implementation might need to be generalized a bit to be usable outside of wdqs.
I am a little surprised such generic library doesn't exist already - as throttling is pretty common thing people do - but if other projects can benefit from our code, by all means we should set it up as a library.
We've already implemented something similar for Gerrit. Making a general solution seems more work than addressing each use case separately.
There is a project to have a more generic throttling solution, but at a higher level.