The efiboot project contains boot.efi, the bootloader that is used in Intel Macs to boot the system. 

boot.efi is a Universal EFI binary with 2 architectures, i386 and x86_64. It can be used to boot Intel-based Macintosh computers with both 32-bit (e.g., first-generation MacBook Pro) and 64-bit firmwares (e.g., Santa Rosa MacBook Pro).

efiboot requires EFI to work.
Computers that do not have EFI need to use other bootloaders, such as boot.
To use efiboot, you need:

• An Intel-based Macintosh (or another computer with EFI that can read HFS+; no such computer is known)
• A HFS+ volume containing a Darwin system
• efiboot
• bless

To make a volume bootable through EFI, efiboot needs to be installed.
Simply copying boot.efi, however, is not sufficient.
To make it work, there are basically two options:

• Blessing the volume
• moving boot.efi to a special, UEFI-specified location (to be verified)

Assuming your Darwin volume is mounted to $MOUNT, do:
"$MOUNT/usr/sbin/bless" -verbose -folder "$MOUNT/System/Library/CoreServices" -bootinfo -bootefi
The above command implements an Xinfo cache for PowerPC processors which is not needed in the case of an Intel-based Macintosh.

For PureDarwin a simplified command can be used that will bless for only EFI based Macintosh computers:
"$MOUNT/usr/sbin/bless" --folder "$MOUNT/System/Library/CoreServices" -bootefi --verbose

The volume should now be bootable.

The firmware of Intel-based Macintosh computers implement sections 3.1 and 3.2 of the UEFI 2.0 specification. UEFI 2.0 defines that the firmware looks for an EFI bootloader at the following locations:

• /EFI/BOOT/BOOTX64.EFI
• /EFI/BOOT/... of FAT16-formatted volumes. Whether this can also be used to boot a Darwin system from a HFS+ volume needs to be verified.

EFI emulation in VirtualBox 4 is capable of loading and executing boot.efi as the bootloader. Hence boot-132 or its derivatives are no longer needed in VirtualBox. We were able to boot PureDarwinNano.iso this way.