Why are you hoping for an RCE PoC? Curiosity's sake? Or because you'd like to use it offensively? The existence/publication of such a PoC makes the world less safe.
As a practical matter, the impact could be pretty low. Very few things would accept brotli encoded data from a client. A PoC also doesn’t have to be for any particularly common configuration.
I’m someone who really enjoys reading Google’s Project Zero blog posts where they regularly walk through building exploits for phones and browsers. It gives you an appreciation for what is possible.