Ah, I see the use case now. I don’t know if you can do that from SFTP though, I wouldn’t even try. Any chance you can do all that via WordPress? you can create users on WordPress and give them the rights to upload media. I know that WordPress permission model is not sophisticated enough but maybe you can trust your users not to mess up with the rest of your content?
Otherwise, I’d explore using DreamObjects although that’s not immediate either… and I’m not sure the plugin you’re using supports S3-compatible object storage.