As I understand it, the Particle Ethernet Feathering is intended to have a small add-on card which will enable PoE. There are three female pin headers right behind the RJ45 Jack that will accept this add-on card. This card hasn't been released yet and I haven't looked at the GitHub in a while to see if that schematic exists yet if you wanted to build it yourself.
This post has more info regarding the PoE support: