Rather than just re-adding HOMEBREW_PREFIX/bin if it's missing re-add
everything from HOMEBREW_PATH. This works well with or without
environment filtering being enabled but with environment filtering it
ensures that ENV/std just builds on the original user environment.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.