Hi,
What you need to do is set up a firewall or ban on all incoming DHCP requests from 00 80 64* – What platfrom is the PXE server on?
Because its a UDP DHCP braodcast the blocking service has to be on the RADIA box. 3COM made a DHCP proxy service that did this and there are others on the net to do the same.
As for ftp upgrades this is more flexiable than PXE. Bundle the image in Rapport and then send it down. Other option is get the primer image and bundle up the extras.