This schematic over at ON Semi answered one question about the design: how to pass the Ethernet signaling on to the device. The secondary side of the magnetics goes directly on to the output RJ45.

Most of the schematics for PoE adapters out there also show flyback power supplies rather than buck converters. They're more complicated, but they are better isolated, and the spec does call for isolation, so maybe that's the way to go. The good news is that flyback converters can run just fine on DC. In fact, when they're used in cheap wall warts, the incoming AC is usually just directly rectified and the flyback transformer provides the requisite galvanic isolation along with an optoisolator for feedback.