@lanodan that shebang does basically two things

dependency resolution, so well, depend() works and starts stuff it depends on

then call the actual script through /lib/rc/sh/openrc-run.sh (soon to be /usr/libexec/rc) which defines the helper functions (like default `start` and `stop`, loading the supervisor=...) and such

and portage would be similar, since it already has /usr/lib/portage/python-*/*.sh scripts, so if we do that with bash already, similar would be posix sh defining helper functions there