I thought they already did something like this. I recall some people complaining because the naive decryption process would double the required hard drive space while the bundle was unpacking on release day.
Pretty sure they already do it. I've also noticed that if you have a fast connection (like 1 Gb/s down), it might be faster to download the game when it releases than it is to preload it and then decrypt it, since the decryption process is quite slow.