I got curious and rechecked this -
Here is grub.efi booting from usb msdos drive with hfsplus partition -
sh-3.2# diskutil list /dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *1.9 Gi disk1
1: Linux 1.2 Gi disk1s1
2: Apple_HFS disk1s2 572.6 Mi disk1s2
3: DOS_FAT_32 fat32 100.4 Mi disk1s3
sh-3.2# bless --folder /Volumes/disk1s2 --file /Volumes/disk1s2/grub64.efi --label "msdoshfsp" --setBoot
sh-3.2# bless --info /Volumes/disk1s2 finderinfo: 2 => Blessed System Folder is /Volumes/disk1s2/
finderinfo: 85 => Blessed System File is /Volumes/disk1s2/grub64.efi
finderinfo: 0 => Open-folder linked list empty
finderinfo: 0 => No OS 9 + X blessed 9 folder
finderinfo: 0 => Unused field unset
finderinfo: 2 => OS X blessed folder is /Volumes/disk1s2/
64-bit VSDB volume id: 0x795006DDA23084CA
--setBoot gives deafult fast boot to grub menu, but when booted that way, grub can only see its own disk (hd0)
If booted via Option key or rEFIt all drives are seen and bootable.
On the GPT drive, an MSDOS partition table is generated from gpt tables by the Mac OSX Bootcamp utility which creates a fat32 partition for the Windows msdos compatible mode.
Or the msdos MBR can also be checked and generated by rEFIt (used by apple intel linux users).
The list of drivers for minimyth seems to include those used for sata on my macs, but you may have better info.