I don't know enough about grub4dos to suggest a specific solution, but
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map --mem /ubcd/images/fdubcd.iso.gz (hd32) map --hook map (hd32) (cd) map --hook root (cd)
I'm sorry I can't be more specific (like for example about the exact meaning or the exact consequences about each MN pair of values).
Since we are talking about at least 2 images (fdubcd.iso and fdubcd.img), maybe more "map" and more "hook" commands are necessary.
About testing with SYSLINUX/ISOLINUX, are you already using v.4.06? If it doesn't work as expected, maybe the new ifmemdsk.c32 syslinux module could be helpful? (I haven't used ifmemdsk.c32 yet, so I am not sure if it is actually useful to solve the current situation.)
I don't like being so vague, but I just thought that maybe thinking out loud may trigger some idea from someone else.