That said, would prefer not to be using nginx, so if anyone has any recommendations for high quality zero-downtime-reloadable HTTP proxies, I'd be very interested.
Looks like bamboo encourages folks to use this strategy via a custom reload command: https://github.com/QubitProducts/bamboo/issues/152
That being said, you can turn ingress into egress with an ifb. A coworker of mine created a proof of concept for using our strategy with an external facing load balancer but I don't think he ever tried it in production so I'm not sure how well it works.
To be fair I'm scared of both the iptables and tc solution on external LBs because you might never know that you're refusing connections accidentally. Linux 4.4 is coming with some patches that should help with this and afaik BSDs have done it right since the start.
Yes, though a bit of advanced planning and creative configuration can usually work around this limitation.
Making the assumption that new servers and services occur within a predictable range of IPs and ports (or have their own haproxy to route within the server), you can create a list of backend servers which are all disabled and just waiting to be individually enabled.
I've done this in the past, and the disabled server entries never seem to have a negative impact on HAProxy. It also has the advantage that you can set your HAProxy instances to peer with each other, so the change only has to be made once to propagate to connected instances.
Disclosure: I'm the author