So I send the address 2BTC, and it explodes?
a zero trust web interface to bitcoin could be an interesting idea - you can perform any transaction on the website , but all the keys and signing is done locally via script+HTML5. but then backups and lost wallets will be a pain in the ass
edit: can you set up a script that will require the site's key to use the BTC? so he will have 50 BTC in the account but can use only one per month or something
Take a look at my project, BtcBuckets. You can limit the size of the bucket and have any spillover sent to any address you want.