Sounds like an interesting idea! Does Duplicacy have an API for backends defined already? I had a squint at the source code and it doesn't look like it.
I do note that duplicacy supports webdav and rclone can serve it with rclone serve webdav that might be a way forwards that is more efficient than using a mount?
Can you see if that works? If it does then we can think about how to make a slicker integration.