From the interview I recently did with David R. George III for TrekCore.com (in which he confirms that the "Lost Era" to which he refers is in fact the one that had previously been titled "The Lost Era"), he also had this little tidbit to offer:
Still possible to be about Captain Demora - she took command of the Enterprise in 2311, which is pretty much right in the middle there. (If it's even about the Enterprise that is - there's still plenty to do in that time frame that doesn't focus solely on the Enterprise.) Either way, though, I'm excited.
DRGIII is quoted above saying it's about a captain he's written about before and given the time period, I'd say there's a very good probability it is about the Enterprise B.