Aside from this being a very useful plugin, this was a test project to learn how to build rebar3 plugins. However, using the generator I overlooked would have done most of the work (aside from the flamingo). 🤷♂️
$ rebar3 new plugin rebar3_flamingo
One thing I did figure out that might be useful in lieu of the guide I planned to write: to test a rebar3 plugin, add it to its own project_plugins, then symlink _checkouts/rebar3_flamingo back to ...
Now you can call rebar3 flamingo from its own project root.