Looks like you can start a Unity Editor instance in batchmode, which is a headless mode that allows you to run Unity without the graphical user interface. This is useful for automating tasks, running scripts, or building projects from the command line. I want to try to make a script that you can bundle your VRC avatar without opening the whole editor.
VRC SDK is closed source (as well as the whole game, duh), so looks like I'll have two options: Reverse engineer the SDK, or import the SDK as an assembly on my scripts.