I have searched all over, and can't seem to find if it is possible to access "My Photo Stream" photos via finder. It seems that this used to be possible with the previous incarnation of Photo Streams, but I can't find anything dealing with the new version.
I'm currently using a tool called photostream2folder, which works great, but it is a black box. I'd like to do some scripting of the process, and for that I need to know how to access these photos directly from the stream.
It seems that this used to be the path to use "/Users/username/Library/Application Support/iLifeAssetManagement/assets", but it is no longer the case.