My solution here was to make a module that puts a "Site Settings" item in the CMS main menu. In there I had a form that saves everything to Site Setting data objects, and made a Page_Controller decorator that fetches them for use in the templates. I used it to set things like contact details, "global" meta data, taglines and whatnot.
I think it would make alot more sense to have a settings page like that at the root of the Site Tree, from a user point of view, but I had a look at the files there once and it didn't look like a fun thing to try hack.
Of course if you're using 2.3 you could manage settings with a ModelAdmin page but it's not a very user friendly interface for someone who just wants to set one or two variables.