Correct, this functionality does not currently exist.
However, I’m implementing it as part of my work on an open tunneling protocol:
https://forum.indiebits.io/t/toward-an-open-tunneling-protocol/30
The goal is to design a protocol that will allow client and server software beyond just boringproxy to be compatible with each other. I’ve already made a lot of progress with the initial prototype for boringproxy, but it’s turned into a bit of a different rabbit hole.
I’m hoping this will be resolved in the next month or two, but no guarantees.