Any way to force AHCI mode on ICH8?

[Morgan Wesström]

I run FreeBSD 8.1 on an old Asus P5B-VM motherboard with ICH8. Its AMI
BIOS lacks an option to enable AHCI mode. Intel's datasheet for the ICH8
family specifies that this feature exists on the ICH8, and the option is
available in the BIOS for the identical (apart from form factor) P5B
motherboard.

http://www.intel.com/Assets/PDF/datasheet/313056.pdf

I've contacted Asus support for an updated BIOS but I don't have much
hope I will ever see one. Would it be possible to patch the FreeBSD
kernel to enable AHCI mode somehow during boot?

Regards
Morgan