So immediately after hacking together something with TrueCrypt (described in my my previous journal entry) and realizing how unworkable it is with side-by-side Windows systems, I set to work looking for anything more elegant, and as it turns out, Windows 8 comes with one: BitLocker.
BitLocker is the file/disk encryption feature introduced as a part of Windows Vista, but it's only available in Professional, Business, Enterprise, and Ultimate editions of Windows. It's available to home users starting with Windows 8 because the edition preinstalled on new PCs and sold in stores is Windows 8 Pro. Windows 7 Home Premium can't use BitLocker to manage encrypted drives, but it can still access drives encrypted with BitLocker.
That means BitLocker can be used not only to encrypt the Windows 8 system drive (partition, really), but it can also be used from within Windows 8 to encrypt a Windows 7 Home Premium system drive if both are installed on the same disk.
So here's a better encrypted triple-boot howto: Encrypted PC Triple-Boot with BitLocker and LUKS Howto.