I have not used any of the BSD distributions for a while but it is my understanding that FreeBSD has support for GPT since 7.1. Converting to an EFI based machine would simply require replacing the FreeBSD boot partition with an EFI system partition (ESP) and EFI boot code for FreeBSD x86 would need to be added.
IA64 BSD EFI booting is supported but from what I can see only experimental support exists for EFI booting X86 or X86_64. Some people have managed to do it using rEFIt on Intel Macs.
A better approach might be to use TianoCore DUET to boot an EFI-based GRUB loader which in turn can be configured to boot BSD. Here is a
Using UEFI DUET to boot Fedora 12 which shows you how to UEFI boot Fedora 12 using DUET. Just add the necessary GRUB stanza for your favorite BSD distribution and it should boot.