Thank you. I began the application process back in November; however, the slow pace of the process is largely because of Japan's Immigration Bureau. The thing is, I think that most of Yamasa's programs start as often as every month. The clincher is whether or not you'll need a student visa (shugakusei). Student visas are only issue for April and October commencements, but other visas for shorter stays are available more often. I don't really know any details beyond that, but Yamasa's website does have a lot of information.
In short, though, I think there's at least a pretty good chance that you can still apply for a summer program.
Let me know if you have more questions.