Standard jobs provide API endpoints for inspecting job statuses from other applications. This would be extremely useful for multijobs as well, particularly if there were a way to retrieve the build numbers of the downstream projects that were triggered from a given multijob top level build.

Ie http://jenkins/job/MultijobJob/5/api/json would return a basic json report including the same info that is displayed in the web interface when visiting a build status page.

Should also work for /lastSuccessfulBuild, etc as well.