Accessing via webDAV is definitely the way to go. Your service could access to external storages transparently, and take advantage of additional services ownCloud could provide, such as encryption or antivirus (if they’re configured)
Note that the “local” external storage is dangerous if it’s misused, that’s why it’s disabled by default. It could be fine if you set it up to access to files out of the reach of the web server, but I don’t think this is your goal.
Technically you can mount any place in the host as long as it’s accesible by the web server.
You could mount the root, so anyone who can access to the configured local external storage could see and download any file in the host, potentially getting access to sensitive information.
You could also mount the web server location, which could potentially allow you to change the code by uploading modified files there. This could easily break ownCloud, or be a huge security risk.
Obviously, you can use it properly, and mount folders such as “/opt” or “/mnt”, assuming they’re prepared to be used as such. On the other hand, it’s weird you need to access to the local FS out of the ownCloud’s data directory, and if you do, it might be a better option to use SFTP or SMB protocols and use specific accounts as if you’re accessing remotely to those files.
I indeed believe that it is a limitation of plex (with VLC no problem). I did not yet try it myself, but that is the information I found on the internet. I’m currently waiting for some new hardware, when this arrives I will give it a try.
I tested this out. Getting access to the plex media over webdav is not possible. It seems indeed like he needs te download the whole files before even being able to add them to the library. I guess this is a limitation of davfs2.
With the local external storage it seems to work fine.