I think the thing is to be careful with what you mean by "prototype" - very few of these ponies which we see could be classed as that. Most of the non released ponies we see are "frankenponies" - simply part ponies - heads, bodies, symbols and hair colour randomly put together in test runs or in error or outside the factory. These are cool and lots of people collect them but they are often not more than someone could do with some rehairing or popping one pony head on a different pony body.
There is the odd exception - Minty, Sweetie Blue, transparent/glitter ponies - they could probably be classed as "prototypes" in a very loose way as could some ponies which were found only on Taobao which have slight variations to their actual release - such as the larger eyed Drama Pony. These are complete ponies which were clearly created in factory for a potential purpose/release or prior to an actual toy run.
Mrs Cake and the magenta coloured baby we have seen, right now we don't know if they will be released or if they are prototypes. Just because things are on Taobao doesnt make them a prototype - just about every release we have seen so far has turned up on Taobao before showing up on our store shelves. Personally with the size of release they have on Taobao I think they ll get a release but we wont know for certain until we see them in box or until they vanish from Taobao completely.
Strawberryreef.com has a lot of the oddities we have seen but so does Bright Idea's blog
http://mysteryponies.wordpress.com/For information on how to order on Taobao you should do a search over on the Trader Support forum as I am sure there will be information there.