Nice, but shouldn't the behaviour change be behind a config setting? And it's not clear what the intent of the change is. Implementing PING/PONG seems different from what you said you were trying to do. And it's section 1.8 of the OpenSSH [PROTOCOL] reference, not section 1.9.

But... before you think I'm trying to be negative... good on you. I wish you well. Getting crypto/security code into open source projects can be a slog as people frequently come out of the woodwork, so don't get discouraged.

And the more I think about this... there's plenty of examples out there about doing HTTP based reverse proxying, but essentially zero for SSH proxying, so if you do that, it would make a great blog post.