Is Arduino Keyboard in Single User Mode Possible?
OS X requires a "Boot Interface" compatible HID profile for a keyboard to work in single user mode (it has only loaded very rudimentary drivers by that point).  The Arduino doesn't provide that interface, so can only work with the more advanced drivers the OS provides once booted.

You can try setting the "boot interface" flag to 1 in the interface subclass (here, I think, changing HID_SUBCLASS_NONE to HID_SUBCLASS_BOOT_INTERFACE) though I have no clue what the actual effect of that will be given that it's a composite device that may or may not conform to the boot interface standard.