As promised I'm going to start (and largely complete - knock on wood) the REST API for Vespene next week.
Authentication will initially use user accounts, so setting up an API account would be as simple as making a new user. I don't plan on allowing any anonymous access right now.
All the normal permission setups will still apply.
The first step will be to supply read access to everything, then we'll do triggering of builds, and then work on making it possible to create, edit, and delete other things.
There won't be anything more to install, you'll just need to upgrade the code as normal.