Several of the newer Chromebooks support USB-C charging
Sure, but I'm not looking to get a newer Chromebook; I like this one, because it's tiny.
I realise that having a tiny Chromebook and then asking about an external battery pack might seem like a contradiction. Here's my use case:
When I'm on call (pager duty), and I want to go out, I usually just throw my phone and my Chromebook into a bag. However, sometimes, the Chromebook's got a flat battery (because either it isn't always going into standby properly; but that's another story, or I'm simply misunderstanding whether it is supposed to be going into standby).
At that point, I want to throw some kind of external battery in there, so that I've got juice. The Anker powerbank works fine with the phone (and with tablets, etc.), but not with the Chromebook.
Actually, thinking about it some more, I don't necessarily need the Chromebook at all. All I actually want is an SSH-capable terminal with a reasonable keyboard, and a battery life that I don't have to worry about. Maybe a smallish tablet with a proper keyboard stand would work better...