Seemingly none of the other kids want to play with me.
I did a few more tests.
Windows 7 disk management will "blank" first 440 bytes of the MBR and will make first GPT partition starting on LBA 128 (on the small test image I made), this would likely depend on the known Registry settings in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VDS\ALIGNMENT
Which normally are (for disks below 4Gb in size aka "LessThan4GB") 65536 bytes (please read as "LBA 128 for the first partition") and 1048576 (please read as "LBA 2048 for the first partition") for bigger disks (aka "Between4_8GB", "Between8_32GB", "GreaterThan32GB").
In real life, I doubt that anyone will have a GPT disk in size less than 32 Gb, so using the area from sector 63 up to LBA 2047 is "safe" and can be a good way to make - even "temporarily" a GPT disk bootable on BIOS.
We could in theory extend the image a little bit, having it begin on sector 34, as the sectors occupied are normally:
LBA 0 <- the MBR
LBA 1 <- the EFI PART header sector
LBA 2-33 <- the actual GPT partition entries
But - since grub4dos in it's latest releases "understands" EFI partitioning (seemingly fine ) - it would be smarter (for a "permanent" BIOS boot GPT disk) to create an actual GPT partition "covering" the range 63-2047, something that can be easily done with GPT gdisk.
This would avoid the need for a grldr with a "modified" embedded menu.lst.
To make it "standard" we could use the gdisk partition type EF01 (which would allow to distinguish it from a EF02 or "GRUB2 BIOS boot partition" ).
gdisk32 \\.\PhysicaldriveN x l 1 m n 1 63 2047 EF01 p v w
and then dd the two images attached to previous post (and later replace the "modified" grldr with a "normal" one).
Since we don't normally really *need* the whopping "almost 1 Mb" volume size , we could even reduce this partition to the range 128 to 2047, which would be still large enough to hold (say) a grldr, menu.lst and (say) a BOOTMGR+\boot\BCD.
I'll have a look for an alternative to the "hacked" MBLDR:
that I used, so that we can get rid of the need to press ENTER to boot (though all in all it would come useful if we add to it a message warning that the disk is being booted in BIOS mode and not in EFI/UEFI )