You shouldn't have to do anything to your Mainstreet platform as the piece of track found at the back of the Fantasyland platform is meant for some collectors to eventually install a second train track that would hook to the now unused straight piece of track at the front of the Mainstreet platform. This is what I gathered from talking to Travis at one of the signing events. I would imagine that you would need some sort of makeshift platform extensions to do so, or perhaps there will be future platforms for the rest of the park,
If you go here: Olszewski Disneyland miniatures - July 18, 2010
you can see pictures of the prototype Fantasyland platform and how it will connect to the Mainstreet platform.