I have the PowerCore 26800 in my possession now, and my employer wants to hand me an X360 before I leave for the middle east in a few days. Very short notice. So long as this works, although slower, I think I a should be ok for the time being.
So I just got off a chat with HP and this is what they said.
System: "Hello! My name is Kendra. I've read your question and am ready to assist you."
Jeremiah Sams: "Can the following model be charged via the USB-c port? HP Pavilion x360 Convertible 15-br077nr 15.6" 2-in-1 Laptop Computer
Agent (Kendra): "Let me check this out for you."
Agent (Kendra): "I am happy to inform you that you can charge this laptop via USB-C port. You can check this link for more information: https://support.hp.com/us-en/document/c05530677"