This project is still fairly dormant, but the 3B+ has given it a little kick in the pants.

The 3B+ includes a 2x2 .1" header on the board that (ostensibly) has the center taps for the four magnetics. You just pass each pin through an AC pin on two bridge rectifiers that have common + and - outputs and there's your 48 VDC power input (there's more to it - you need to properly engage the 802.3af protocol to actually get that 48 VDC, but that's the job of the TPS23753A, of course).

The 5 volt output would be fed back to the Pi on the GPIO header (the same way Pi Power does it today), so the whole thing winds up being a Pi Hat.

To be fair, the Pi Foundation has already announced that this will be A Thing very soon, so there isn't a huge amount of point to continuing on, other than learning something and/or the typical "because it can be done" hacker mentality.