In Caddy we've implemented "dynamic upstream modules" which allow an arbitrary (compiled-in) plugin to give the reverse proxy a list of upstreams that can take the request. A pull-based mechanism could relatively easily be constructed with this feature: https://caddyserver.com/docs/json/apps/http/servers/routes/h...

I could imagine a dynamic upstream module that receives UDP or TCP packets from backends with a number estimating how many connections it can handle at that time. The module then tells the reverse proxy which upstream has the highest number and the reverse proxy selects it.