While reading J.A. Konrath's latest blog post, I noticed he created a new Google Play Books Partner Center account. After acquiring Oyster's expertise and nearly 9 months with closed gates, it looks like they're open for business again.
Kudos to Google and Oyster for re-opening an important eBook retail platform to new publishers!
Update - Mar. 11, 2016: It seems Google closed Google Play Books Partner Center for new accounts again. They were open to new accounts for at least a week, but I have no idea which date they closed again.