Can I backup an arbitrary file on iCloud?
Yes you can — iCloud can be used to sync anything that you wish.


Open ~/Library/Mobile Documents and create a folder (call it anything you wish).
Inside, put whatever documents you would like to sync over iCloud.
That folder will sync with other Macs (and jailbroken iOS devices, and on the web too if you have a Apple Developer licence).


Alternatively, you can symlink a folder on your Mac in to the Mobile Documents folder so that the files remain in their original location.