I have read your posts over the last couple of years on different boards. You know what you are doing and it shows.
However, your under UPC as was I until just recently. I'm now under IPC.
Running the T&P outlet outside is a great idea, but there needs to be an air gap fitting installed at the tank so a person checking the tank can see if the T&P is leaking. UPC does not address this unless it is in coding after 2000.

Keep up the good work