No real offical method in order to PXE Boot a PXE REP you would just have to have multiple PXE reps on the subnet, so that one of the other PXE reps would be able to process the PXE request.
You must have another PXE Rep in the subnet in order to PXE boot your Rep.
The problem that you run into is that without any other PXE Reps on the subnet, there will not be any machine that will "hear" the PXE DHCP requests when the machine boots. Each PXE Rep has two services installed. The LANDesk PXE Service and the LANDesk PXE MTFTP service. The PXE service essentially "listens" for client PXE DHCP requests that are broadcast among the local subnet. Once that request is received, the PXE Rep then negotiates with the core to determine the boot order, and then responds to the client machine with a DHCP response pertaining to the PXE options.
If you boot your PXE Rep and there are no other machines out there with the PXE Rep software installed, specifically the PXE service, then nothing will happen. The tree falls in the woods, but no one is there to hear it.