IMHO more "proper" or rather interesting approach, could perhaps be some kind of helper program - "injecting" "alternator" sub-process - which would be running under s6-supervise.
The haproxy people have already done this work for you. They call the program haproxy-systemd-wrapper. It's undocumented, naturally. There's also a haproxy-upstart-wrapper.
* http://www.serverphorums.com/read.php?10,649689 * https://unix.stackexchange.com/questions/239805/ * https://github.com/wankdanker/haproxy-upstart-wrapper
