You won't be able to do it with scripting even if you could take amounts into account, because the script for this transaction would have to affect all FUTURE transactions from this wallet. This is fundamentally diferent from what scripts do today.
I think there would be a way to do this if you could take the amount into account. You'd send a big transaction to yourself in the amount of your wallet, but that transaction would only allow spending of itself according to the script.
You may be right that it is too big a change to introduce at this point. On the other hand, you have to consider how common things like the allinvain heist would become if bitcoins became mainstream. Unless wallet security is simplified vastly simplified, the VAST majority of users will be insecure. Suggestions for USB drives in safes and wallets printed to paper abound on these forums. My sister won't ever do that, not to mention my parents or grandparents.
We're just at the stage of discussing the underlying technology. There are lots of ways to package it and make it easier to use. Your grandparents won't have to understand how ECDSA works in order to send some coins, and they won't have to understand these protocols, they'll just have to insert a smart card.
Maybe you could do this independently of bitcoin. Somebody could start a web service that provides you with a public key. You encrypt your wallet as usual, then you encrypt the wallet with the public key. When you want to open the wallet you send it to them and have them decrypt their encryption (or they just send you the private key).
But they don't do it immediately. They send you emails and text messages that someone's trying to decrypt your wallet. Only after a day, or after you've proved in some other way that it was you, do they send you the partially decrypted wallet.
I like this a lot. It may well be the best we can do. Certainly a lot simpler than a live distro with a wallet on a USB key in a safe. But it's a pity that we lose the ability to do small transactions without hassle.
You probably wouldn't do this for small transactions. You don't mind carrying around $100 in your wallet, and you won't mind carrying around a few bitcoins on your mobile phone, or in a bitcoin client with default security.