I'm using PXELinux so I'm not sure if this will help. I was getting a headache setting all the flags etc to boot winpe from files. So I told it to forward the iso instead. I'm not familiar with macs. If there is something like a pe disk you could theoretically send the pe disk to the mac and theoretically have it establish either a nfs or smb share, then install over that.
My winpe section just sends the whole 300mb disc over.
LABEL winpe
MENU LABEL Windows 7 x64 Installer
KERNEL /memdisk
INITRD winpe_amd64.iso
APPEND iso raw
I was under the impression that macs were bsd like, so it could possibly work like a linux pxe share. I got most of what I needed from this site -> http://www.serenux.com
Links
http://www.serenux.com/2010/05/howto-setup-your-own-pxe-boot-server-using-ubuntu-server/
http://www.serenux.com/2010/05/howto-get-an-ubuntu-live-cd-to-boot-off-a-pxe-server/
I used my router to redirect the pxe to my server. Not sure if any of this would help, it's just a start.