sadly I found the thread this late. last week I worked on exactly the same build plugin and I experiensed some difficulties as well. I wrote the same callback version as @sacha, but it didnt work (race condition during the build IMHO). the next one I wrote was a future powered version (since the build process uses fibers to parallelize its tasks) - I didnt notice an inconsistency so fare. But this is not the way its supposed to be done - some thoughts about it
since @sacha s repo is offline (or in a place were I couldnt find it) maybe some of this could be helpful to someone.