Yes, backup on demand is not so hard work, but now it can make more questions, then answers.
- Client must choose what to restore (GUI changes)
- Reseller or admin must choose how much disk space it wil need to make for backups (it's better to know how many disk space use only site, for example some of my clienst stored there about 40 GB of data).
In this case we must rewie the backup engine, don't hurry. I think it must be only in 1.2.0
Backup on demand button will only trigger the backup script for customer.
Disk space for user should be based on customer disk space limit (no space for backup then, no backup). Now, if your user are storing about 40GB, it's your problem. You must fire them about your rules or downgrade their space limit to avoid such thing.