LUKS can already be used to support boot-time encryption. It doesn't support full disk encryption, and the boot partition is not encrypted, but the OS is. I actually manually did this in a version of Ubuntu using these instructions. There were a couple issues with the instructions, and I wasn't doing it for dual-boot, I was doing it for an older version that didn't support it in the install process, but I can confirm that it does work.
ETA: It is possible that you meant full disk encryption (so there is no unencrypted boot sector). It is also possible that you meant encrypt-in-place, which is not what I did (I encrypted pre-install). To that end, I agree that it would certainly be nice to be able to encrypt a system in place using a GUI front-end after the OS is already installed.