I need this too, because my MacBook 2007 version cannot boot from an USB flash drive.
From this guide http://www.supergrubdisk.org/wiki/SGD_Howto_make#Gnu.2FLinux_.28and_being_usable_for_storage_later.29
It apears sg2d relies on MBR boot code thus you would need to backup the boot code of a sg2d drive and the partition table of your harddrive, then merge the two and write it back to the harddrive like the guide shows.
I have not tested this myself. If you choose to do this it is advisable to backup the data of your harddrive. This will overwrite the existing boot code of the harddrive needed for OS's like Windows!