I write an exporter, which exports PBR materials, mostly from the PrincipledBSDF shader. Would like to add a small directory of generic environment textures in my zip file, in a format that Blender does not use. Like a 2 umbrella studio environment texture, seen here as a reflection texture.

User could then also pick from a custom property list, in addition to a the .hdr in the world node. The file would get copied to the export directory.

Problem is how do I know where the directory from the addon is, in a completely OS independent way?

This gets the root directory of the add-on from any file (or at least those in the same directory). Can always do an os.path.join() if I need a sub-directory.
def getAddonPath():