Multisite: Per Site User Reigstration?

I’ve done quite a bit of googling. I’ve found threads as far back as 2-3 years with tons of people saying, “omg I wish, and I hope it’s in the next version”. Here we are… 3 years later, and I can’t find anything that works like this. It’s not a feature in the core, and no plugin exists.

I have a WordPress multisite install with 3.6.1. I want users to be able to sign up for a blog on my main site: http://example.com and get their own blog: http://blog.example.com and domain map: http://www.their-blog.com/ which they can already do. This is awesome. I can even charge people for their blog and let them pick their own themes and plugins.

However, I do NOT want the “register” link on their blog to send them to my site: http://example.com/wp-signup.php… This is terrible. I want the register link to send them to a register page on THEIR BLOG that looks and acts just like THEIR own WordPress install. I.E http://www.their-blog.com/wp-signup.php

The problem with this is, if I have a lawyer, chef, dog walker, porn star, and the pope with a blog on my network, when users register on their blog they will be sent to a signup page on the core website. This is fine in many instances but I need each of these sites to act separate. Mostly because my core website will be selling blogs and site registrations. When a user wants to signup for pornstar.com or thepope.com the last thing they want to see is a completely different theme, and website, and upsells to start a blog. This is also the last thing my customers will want to see their users experiencing. It’s a lose/lose situation.

So, does anyone know of a way to fix this without having to manually hack and code each sub-site?