I found a nice thread on the "Terabyte" forum about "truly" hiding a GPT partition. Terabyte are the creators of "Image for Windows" and "BootIt Bare Metal" (BIBM), which is available for MBR disks, and more recently also for GPT. It's a very powerful product, but I like Grub4Dos better (open source, a bit less intrusive, etc.)
The topic is "Can you really hide a GPT partition?", the link is below, but I'm posting some quotes from there as well.
======== QUOTES ========
"I would like to hide one OS partition from the other. By "hide" I mean prevent its files from being seen or accessed."
"> For what it's worth, Hide does work with Windows OS. NTFS and FAT
> partitions can be hidden.
Not really. Hidden NTFS and FAT partitions are clearly visible in Windows. The only thing you can't see is their contents."
"In my particular case with a single-user machine, my concern is possible harm arising from an inadvertent (stupid) operation on my part or harm from spyware/malware as tas3086 suggested."
"Hello! Can anyone help with hiding a drive, so it is not visible in other windows installation 'Disk Management'? I am testing a demo BIBM in Oracle VirtualBox, and can not get the drive NOT to show up in 'Disk Management'."
======== END QUOTES ========
So, to me this confirms that the "hidden" attribute is not of much use. For me, the current alternative of simply not attaching a drive letter to the "other" partitions is good enough, maybe other alternatives will pop up in the future.
It seemed useful to post this info here.
Quote of the day: "I can't get the drive NOT to show up".