Can I change the location in which the backup is saved? Am I missing it?
You’re not overlooking it. There might be some trickery with a symlink you could do, but I don’t want to post details of something that technical that I haven’t tested.
It’d be helpful for us prioritizing future work to understand why you want to store the backups somewhere else.
Not at the moment. The new App Sandbox in OS X makes this problematic, since it’s easy for an app to lose access to any location outside its own sandbox (e.g., it can lose access when you migrate to a new disk). We want OmniFocus to always have permission to write to its own backup folder, since we want those backups to be trustworthy.
If you like, you could set up a script which watches the OmniFocus backup folder and automatically moves your backups to a different location.