I'd like to see an option, similar to that in the SVN plugin, that lets you add multiple Mercurial repositories to a Hudson job.

I realize this has been addressed in JENKINS-1143 and JENKINS-1344, but I think that this feature should be reconsidered. First of all, as stated in JENKINS-1344, not every collection of repositories will be organized in a forest. The forest extension itself is not bundled with Mercurial and, according to their wiki page, "does not appear to be actively maintained".

http://mercurial.selenic.com/wiki/ForestExtension

So I think that these days (the above issues are 2 years old), most projects will either use multiple but unrelated Mercurial repositories or they will use subrepos (which, I think, are still considered "experimental"), but not the forest extension.