I don’t know for sure on that. Network folder have been hit and miss for people depending on the configuration.  I don’t know about dropbox.

The Photoshop API may block it… or it may timeout trying to open the files. Or maybe it will work. I really don’t know. Try it and let me know if it works. If it doesn’t then I don’t think there is anything I can change in SPA. It would be up to the OS and Photoshop API security to allow the file transfer. SPA has no control over file security.