I asked about this on IRC. I finally succeeded in customizing my SiteConfig in SilverStripe 3.0.2, and I am trying to make it versioned as well. I have the following lines in mysite/_config.php:
My database now has the following tables:
SiteConfig, SiteConfig_CreateTopLevelGroups, SiteConfig_EditorGroups, SiteConfig_Live, SiteConfig_ViewerGroups, and SiteConfig_versions
The SiteConfig_versions table indeed has a couple of old versions of my SiteConfig. However, only the "Save" button appears in the UI, and I cannot save a draft, and there is no preview option to view the home page with the new siteconfig before I save it... Does anyone have suggestions?
EDIT: It looks like I need to write the function "updateCMSActions()" in my CustomSiteConfig extension to specify these actions.
EDIT #2: but all these actions are hard-coded in getCMSActions() in cms/code/model/SiteTree.php, so I need to rewrite as much of that functionality as I want from scratch, since SiteConfig is not part of SiteTree, and in particular, it should not be possible to delete the SiteConfig from the site.