I’ve been working on something similar-- Firebase is awesome for quickly standing up multiplayer stuff.
They have a C++ SDK, but it’s missing the realtime data stuff that’s present in their other SDK’s.
Using VaRest Plugin against their RESTful server definitely would work, but part of the “magic” of what Firebase is lost when you have to manually poll.
I actually just got out of a talk with a guy on their team at SIEGE. They’re about a week out from releasing the first stable version of their Unity plugin. He didn’t seem to think they were working on an UE4 plugin just yet, but didn’t say it was out of the question.
Might be a fun side-project to work on yourself with friends!