For those following along - I emailed C-M support to see if they could recommend an extension cable or adapter. Their response:
Sort of wishing I hadn't thrown out the packaging. Might just bite the bullet and go with an e13ctron case...
Dear Jeremy,
Thank you for your inquiry. Aftermarket charging cables and accessories are not compatible with the iPhone 4 Jett Metal case, so I am unable to recommend any particular dock or charger.