centos - PXE Boot FreeBSD ISO from a Linux PXE server

I have a working PXE boot server running on CentOS 5.5 that works perfectly for any flavor of Linux. I'm trying to add FreeBSD to the install options as a ISO memdisk, but so far nothing is working out. When I try to boot, I get the following:

Loading memdisk....Ready
MEMDISK 3.11 2005-09-02 Copyright 2001-2005 H. Peter Anvin
MEMDISK: No ramdisk image specified

Snippit from my /tftpboot/pxelinux.cfg/default. The CentOS install works fine, but the FreeBSD one fails.

default menu.c32
prompt 0
timeout 300
ONTIMEOUT local

MENU TITLE PXE Menu

LABEL CentOS 6.0 x86_64 NO KS eth0
        MENU LABEL CentOS 6.0 x86_64 NO KS eth0
        KERNEL images/centos/x86_64/6.0/vmlinuz nofb text

        APPEND initrd=images/centos/x86_64/6.0/initrd.img ramdisk_size=100000 ksdevice=eth0 

LABEL FreeBSD 9.0 NO KS eth0
        MENU LABEL FreeBSD9.0 AMD64
        kernel memdisk
        append iso
        initrd images/freebsd_isos/FreeBSD-9.0-RELEASE-amd64-bootonly.iso

I've tried "append iso raw", "append iso" and "append raw", but it does not appear to make any difference. I've also tried using the FreeBSD Memstick .img file and "append raw", but same results.