Looks like s6-linux-init 0.0.1.0 pulls s6 in as a build-time dependency. Not a huge issue, but might be worth updating the docs to clarify that until the next release removes that? I've learned to trust your docs and build tools enough that I spent a while hunting for what I was doing wrong :)
Also, I'm in favor of making the shebang use bindir; in the past, I've hit issues with some packaging validation tools that check shebangs to make sure the desired files exist (namcap in particular). Not the end of the world if you keep it at the current, but would save me some scripted regexing - Les