There is a left luggage facility in the entrance of the park where you can leave bags and there are lockers at the train station too (after an x-ray of your bag though).
It is too simplistic to think it is simply a matter of changing the coin mechanisms and opening the lockers, there ARE security implications. I know bags are checked on the way in but a place to store things in the park, in an area that is quite hard to monitor is a different risk. There is also the issue of liability. I know there would probably just be a sign saying items left at owners risk but this has to be communicated well in various languages.
If I were in charge I would be tempted to convert the current left luggage so that access from within the park is possible but there isn't even the need for that really. Anything you buy from the shops can be sent to your hotel or for collection from Disney Village and picnicking isn't allowed within the park. It also isn't to hard to go out of the park, get your bag and come back in.
I feel I have rambled on a bit too much on a thread that is supposed to be about the name of the park but this isn't a simple issue.