The workaround for source port exhaustion seems obivous: bind lots of IPv6 addresses on the interface that haproxy uses for communication with the backend serve

The workaround for source port exhaustion seems obivous: bind lots of IPv6 addresses on the interface that haproxy uses for communication with the backend servers. They are enough of them available :-). (Or if you use a private network, IPv4 addresses work too, of course).