> 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.