That’s something we left out on purpose because it adds a lot of complexity and many people reported Distillery scripts complexity as one of its biggest downsides. I encourage to build regular scripts on top of the main script, which is more Unix-like.
Similar for boot scripts. If you do need to hook into a particular command, then you can use the env.sh file, but I would avoid these particular papercuts. 
If there is something that is hard to achieve with those approaches, please let us know, and we can see how to tackle the individual problems. I would prefer to move the discussion from “we don’t have this” to “we can’t do this”.
Plugins are done with custom steps. That’s something we had since day one because they are required for projects like Nerves. All of those are documented and used by the community, maybe what is lacking is actual guides to help people migrate from Distillery.
