I am planning to take this route on the opening day, April 15, starting in Matsumoto and eventually ending in Kanazawa.
When I went to the website, I discovered that this year, for the first time ever, it is possible to reserve tickets in that direction, from Ogizawa to Tateyama. Although the process is slightly obscure, the English language site is quite understandable, credit cards are accepted, and a reservation may be canceled without penalty as late as 11:59pm on the day before the reservation.
As of 4pm today California time, all timeslots in that direction were available. I picked 11:30am from Ogizawa. To do that, you have to be on the train leaving Matsumoto station at 9:22 (or earlier). Trains on this route are mostly local and not that frequent.