Well, shortly after my last post three months ago, I managed to break my right elbow. Sooo, six weeks in a cast and another four in physical therapy, I only this week got back to attack this problem.
The upshot it has worked out pretty much as we all thought it might. It did mean removing/substituting hard disks (guarantee be damned) but I've now a seemingly perfect dual boot of separate instances of Windows 7. I've not tried to set this up via easy BCD, but simply go the one-time boot override method from the BIOS. Thus, if I want to boot off the MBR disc I select the Seagate (1) option. If I want to use the GPT disk I simply let Windows Boot loader do its thing from startup.
As for the type of EFI/legacy setup that I've got on my Asus N76, it very much is then 'dual capable'. Again, what is interesting is that my BIOS does not have a 'Launch CSM' option. At least for Asus users, it seems, there is an explication. On my machine with Win 7 Home built in, the choice is 'implicit' and not meant to have a user intervention because it shifts over automatically. If Windows Boot loader doesn't find an EFI compatible disc, the legacy option kicks in to look for an MBR. On the other hand, the Windows 8 flavor of my machine (launched in january), sports the revised BIOS with the 'Launch CSM' option. Now, the choice has become 'explicit'. Why? I can't say, but the reason must be linked somewhere with Microsoft's secure boot business which of course doesn't appear in 7. Any speculation on your part, is welcome.
Anyway, thanks for your interest and participation in getting me to solve this problem, even if belatedly.