A patch that attempts to implement this change. It's entirely possible that I have overlooked something critical, or that I completely went about it the wrong way. Solr tests do pass with this patch on branch_4x.
The little bit of poking around that I did in the UI on 4.2-SNAPSHOT did not turn up any noticeable problems. I just fired up the example. I will be able to try it on 4.1 with my multicore dev index tomorrow.
If I did do it right, or the right way can be done quickly, it would be preferable to get this into the 4.1 release.