If we're not talking about an async model then the suggestion is much less drastic than it sounded at first. In that case the crux of your desire is simply allowing the hosts to signal readiness more directly.

You would almost never actually wait for host machines to dial in. You would have a list of hosts that are ready or not ready as they would almost always be ready for more. You want to assume readiness (as this lowers latency) and feed the fire hose.

But in this interpretation, in a world where an LB would be using an existing connection to host machines with HTTP/3 we're basically already there. I suppose its trivial and standard to signal unreadiness to the LB from the host with a 429 Too Many Requests response code.

Off the top of my head I'm trying to think how a host could actively signal to an LB that's its ready for more requests... I suppose its trivial and common to use a health check. Is it even a change to say that these need to be updated to achieve your goal of host to LB pulling?