Short version is that I agree with your assessment that this is not supported by Apple. Anyone doing this is advised to have a most aggressive and consistent backup strategy. That should be the case anyway with any Boot Camped disk, but all the more the case with this 2.2+TB disk work around for Boot Camp.
Long version is that I think the work around is interesting. I'll bet Apple would consider the work around a bug and may try to close it if they ever figure out it's possible. I say that because Apple's tools, both Disk Utility and diskutil, refuse to produce a hybrid MBR if the GPT contains 5 or more partitions. MBR can only contain four partitions. In a Lion or Mountain Lion installation, three partitions are needed: EFI System, OS X, and Recovery HD. This leaves one partition for one additional BIOS booting OS.
For a good primer, and then some, on hybrid MBRs (vs protective MBRs) see this article by Rod Smith, the developer of GPT fdisk (gdisk) and rEFInd, the EFI boot manager:
Hybrid MBRs: The Good, the Bad, and the So Ugly You'll Tear Your Eyes Out
Bill's around gets around the Disk Utility limitation by creating three partitions, but are actually four due to the EFI System partition. One partition is formatted as a FAT32 volume, therefore a hybrid MBR is created. The installer then splits the first HFSJ partition into two to make the Recovery HD. But instead of nuking the hybrid MBR entirely as a result of a fifth partition being created like Disk Utility would, the installer seems to just drop the MBR entry for the last partition, which is Bill's 2nd HFS+ data partition. Since there can be no MBR entry for it (no more entries, and MBR can't define a location beyond 2.2TB anyway), the main negative consequence is this volume can't be accessed from within Windows, no matter what file system is used.
Normally, if only Disk Utility is used to add another partition to a disk already containing four, like what happens when people with OS X and Windows already installed want another partition for data, the result is the hybrid MBR is blown away in favor of a protective MBR, and access to Windows is lost. It simply vanishes. (Access can be restored with a tool like gdisk, but it's a CLI only application, and exactly what to do in each instance isn't exactly obvious.)
There is anecodatal evidence from these and other forums that upon major upgrades (SL to Lion, Lion to ML) if the disk contains 5+ partitions, and/or the GPT and MBR are not in sync, something during installation causes the hybrid MBR to be replaced with a protective MBR. So again, good reason to make judicious backups especially before using Disk Utility to make repairs or OS updates/upgrades. I don't know exactly what causes this to occur so I can't say exactly what situation to avoid.
Anyway, Boot Camp is a rabbit hole. It gets vastly deeper and more risky with each "unsupported" behavior added on, and this one adds both a fifth partition as well as a disk larger than 2.2TB. Risky in that if anything modified either the MBR or GPT, it'll get messy fast! But at Bill now has in effect backed up both his MBR and GPT partition schemes on Apple's forums so his chances of recovery should something get messed up, are much better. So it's not a bad idea to take a snapshot of your current partition mappings if you go down the rabbit hole.
As for why there's a limitation on disk size: Apple hardware supports Windows through an EFI compatibility support module, called the CSM. It's BIOS baked into an EFI module. In effect when booting Windows, Apple hardware has BIOS. When booting OS X it uses only EFI. EFI specifies GPT partition scheme. Windows booted in BIOS mode only honors MBR boot disks, hence the need for hybrid MBRs on Apple hardware. And MBR encoding only supports disk sizes of 2.2TB or less. So the problem is a combination of limitations. The long term solution is for Apple to support EFI mode boot of Windows, and it's unclear if that will happen as none of Apple's hardware meets the minimum Windows requirement for UEFI 2.x spec firmware or greater. Apple EFI is based on the Intel EFI 1.10 spec. Nevertheless some people are having varying degrees of success getting Windows 8 to boot on Apple hardware in EFI mode. There's a huge thread on Mac Rumors of people working on this.
