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.
But it may cause more bugs first of all. It's only a wishlist. I understand what you want, but I disagree with you. First of all we must concentrate on beta testing and fixing existing bugs, not to open new bugs to your own head. It will provide us to release 1.1.0 ASAP.
Also, for example I said about a problem, that may cause a Critical bug. I have only 2-3 such clients, but some users may have >30 such clients, and they may does't know about it. Don't hurry, we need a review. Also it may cause a server overload.