Very interesting concept. This has also already been implemented in Tahoe LAFS (www.tahoe-lafs.org) but for some reason it was never really adapted for something that scales beyond local networks.
I always thought that this would be an awesome idea since a lot of people already operate their own NAS, oftentimes with spare disk space that could be used as intended by this project, but don't want to pay additionally for some cloud provider for a decentralised backup.
Add a 'high-water' configuration option which, if reached during a file create/copy operation, prompts the user to Authenticate with 2FA and approve the extended budget.
I'd be okay with this feature if I got a 2FA alert that something 'big' was about to happen, and could approve/deny it in realtime when it happens.