I will check with the Ext JS framework team to see if it will stick around. That being said a better way of doing this is by placing a Toolbar in as a dockedItem, setting the dock config to 'bottom' and the ui to 'footer'.
On a different note, when adding custom configurations the designer will automatically assume that it is a string unless you denote to it that it will be some other sort of object. If you put in "buttons: " and then clicked add you would see that it would create the buttons configuration as an array instead of a string.
Thanks for the infos Aaron!
Please highlight this feature for custom configurations in the UI somehow, maybe with a dropdown between config key and add button which defaults to string and has an array and hash option too.