It might be tricky to write a web control panel interface to get the token from, say, Dropbox, to use rclone, but you can run it from the command line--even on another machine--and it is not so difficult by running 'rclone config' it steps you through. In fact, it is worth playing around with rclone a bit on a desktop computer before trying to use it on a headless computer.
Typically the 'token' is a long phrase between curly brackets, and you can copy it from the setup dialog on one computer and paste it into a web interface on, say, the OMV machine.
It would be good if the web interface allowed rclone options such as 'exclude'.
rclone works well for this purpose, in my experience.