The just updated special site for patch 6.1 now says the following for Empyreum:

Thanks to tireless volunteers from near and far, Ishgard's once-ruined Firmament now boasts a host of facilities─including a brand-new residential area, Empyreum! During the first round of sales, empty plots will be distributed via lottery system, so those who yearn to live amidst the grandiose stonework of Eorzea's ice-crowned city-state will want to visit the plaque of their favorite property to enter.
It sounds like they've decided against using First Come First Serve in Empyreum for now. Whether this is going to apply to wards in all housing districts is something we do not know at this point.

We'll have to look forward to the Live Letter next week, and probably a developer blog or news post on Lodestone the following week, for more details.