I may have a temporary work-around for you as you wait for a real answer to your question:
we have turned almost all the way over to LDMS and our exiting PXE server is on its way out...so this means that we have an existing PXE infrastructure that provides us with some stuff that we still need, but we're also using the PXE options available through the PXE REPRESENTATIVE provided by landesk.
The way that we are able to use both the new and the old looks something like this:
A) we have four subnets on our network
B) we have a top level subnet where our old PXE services run and the other 3 subnets can utilize those services
C) on each of the 3 subnets we have a PXE REP that was provided by the script that is included with LDMS 9.0
D) when the PXE reps run, they pretty much take over PXE requests on their respective subnet, though if the PXE REPs are disabled then the old PXE services become available.
C) we can remotely turn on/off the PXE REP services at will with a batch file, they run most of the time to provide functionality with LDMS stuff, but when we need the old we just run the script to disable the PXE REP.
the script looks like this:
sc \\ahsm58p-lib01 stop "LANDesk(R) PXE Service"
This script is a batch file and it STOPS the services; replace the STOP with START and you have the restart version.
I might suggest that you try this option as a temporary work around if you think it will work in your network environment. I think it would work if your PXE REP responds faster than your old PXE server...
We have had to change our PXE boot image for Landesk so many times that I wouldn't bother with trying to integrate it into our old PXE services until we 100% sure that it wasn't going to change again. A recent patch that we applied required that we rebuild our BOOT.WIM file and the easiest way for us to redeploy it was to just reuse the PXE REP DEPLOYMENT script.
-anyway, good luck on all fronts.