I have found that the Apple Store is carying a 3rd configuration for the 13-inch MBP with Retina. (I bought one and am typing on it.)
This isn't a custom order, they seem to carry this configuration at all of the Apple Stores, but don't have it separate on the website... which is a reason why I'd think you'd want to add it - let people know it is available if they go into a store and ask...
How do you add additional configurations like this to an existing product - I'd think it should be easy but...
I'm interested, yet leery because of the HD4000 that is pushing a whole lotta pixels.
Additionally, I'm curious if the 13" retina, like the 15" retina, has the SSD chips soldered on the main board or if it is really a user replaceable SSD.
Would someone please let me know when they find out.