I guess that's why I just changed it in the Berkshelf gem itself. Also, we have workstation cookbooks that keep everyone in sync, so a property in the ~/.berkshelf/config.json would be useful as well.
I just find the Ruby solver is so much better and actually resolves dependencies, whereas the original gecode dep solver keeps failing. There seems to be an issue that cropped up a month ago when one has a private and public supermarket in their Berksfile. The Ruby solver doesn't have any issues with it.