Or, since mover is just a fancy script in itself, there is nothing keeping you from implementing your own script that does exactly what you want since you are a software developer.
It's really not that complicated, the worst part is going to be ensuring that the clients see and work with only the latest copy, and not overwriting a more recent version with your script. All the data can be accessed directly from each disk, through /mnt/diskX and /mnt/cache, the user shares are simply the merged content of the root folders on all the disks.
The reason it's not been done so far is that there are many other ways to accomplish what you are asking instead of breaking the current behaviour of mover which assumes there are only uniquely named files in a specific path on each disk.
Backup routines are plentiful and well thought out, like I said earlier you can copy the data to a second location on a schedule with a one line script.
If you are dead set on implementing this exactly as you suggested, you are certainly welcome to experiment with it, you won't "break" Unraid, but you might get some unexpected results. Unraid is extremely flexible, you can do all sorts of things that aren't officially supported. Have a play and see what happens when you put the same file name on a second disk with the same path. Like /mnt/disk1/test/testing.txt and /mnt/cache/test/testing.txt. When you access /mnt/user/test/testing.txt, only one of the files will be there. If you populate the file with content when you create it like "this file is on disk1" and "this file is on cache", you can see what's happening. If you delete the file shown in /mnt/user/test, the second file will magically take its place, making it appear that the deletion didn't work. You can also use this "feature" to hide files you don't want shown in a user share, simply by manually creating a file with the same same on a higher priority disk. Keep in mind that Unraid sees this as an error condition, so don't expect the existing mover script to play well with it, you will need to make your own mover script.