There isn’t any real support for 3rd party channels at this point, and no 3rd parties have expressed interest. Where you planning on writing a channel? What sort?
I kind of like your idea for allowing them to be in the Application Support folders. But I’m not sure why there would be separate lists for client vs. server. Also, this would add the complication of possible duplicate channels in different locations.
Regarding “client/server”, no good reason to separate those.
I found an existing “…/Application Support/PanoramaX/” folder that has separate “client” and “server” folders – so, just following the existing example.