Thank you. Shouldnt the Shottkey Diode be the other way around ? https://en.wikipedia.org/wiki/Buck_converter

Nice work! I don't think the power supply is needed for many projects (such as the one in the video), so having it there without explaining what it's there for may actually scare off potential hat builders. Personally I'm afraid of inductors :-)

It took a while to find a descrition of using a FET or MOSFET bridge that explied why the voltage loss was nearly zero but here are two:

http://www.thetaeng.com/designIdeas/FETBridge.html

http://www.edn.com/design/analog/4368525/Use-a-self-powered-op-amp-to-create-a-low-leakage-rectifier

They need testing to be really sure they act as the write up say they do.

Another way to hanle it is to put a battery backup on the the board with Velcro mounting the battery to attached by the user so it does't interfere with any parts placement.  A +/- 12 or 15 volt 2/3 amp power supply might be a good deal to add on the battery charger as an option.

Shouldn't the terminal be on the other side of the board?

I used the back-powering protection circuit that you have in this HAT, which is similar to the Pi HAT example specification (however a change from a P to N MOSFET) but I have too much voltage drop coming out of that circuit.  Any ideas on that ?