PXE Reps, deploy em on each subnet. Make sure none of them are across WAN links or already advertising PXE through DHCP.
May have to coordinate with your network teams to make sure they are not going to shut you down during the process.
It is probably fine to deploy a PXE Rep accross a WAN link. I think what Thomas meant was that you don't want to pull an image accross a WAN link.
If you have a remote site with a slow WAN link, pulling an image over it could take it down or slow it down.
You should somehow get a UNC share at the remote site to store the images to prevent pulling images over the WAN and have scripts for that site to pull its images.