I can confirm that this gizmo does passthrough charging: https://www.amazon.com/dp/B0711NSNQL/ -- it's been running my Dreamcatcher board for a few hours while I've shifted plugs, carried the whole thing around the house, inserted a USB ammeter in the upstream line, etc. All perfectly uninterrupted.
Down-side is that it gets pretty hot while eating 5+ watts from the charger, passing ~3 to the dreamcatcher, and storing the rest into the batteries. Hot enough to start melting the plastic case... yikes! Note to self, don't set the thing on top of a blanket next time. It needs airflow!
I like the idea because I can use it as a charger for topping-off my 18650's from any USB source, or use it as a power bank if I find that I have charged 18650's and a USB device that needs powering. Bonus, the little LCD shows the current drawn from each USB-A port, though it doesn't also show how much it's drawing through the micro-B input.
Note that the batteries are not included, so the price is actually pretty terrible unless you consider that a feature. But I figured I'd mention it here since the general style is interesting, and there are versions with varying numbers of battery slots, which might make it applicable to different applications.