This is a quick recap of the recent discussion on the ltsp-discuss mailing list.
This may only be applicable to Debian and Ubuntu implementations of LTSP.
If it is desired to boot LTSP client from one server, and mount the root/swap NBD image(s) on another server (or a virtual machine), the following may be placed on the kernel command line, whichever way it is supplied (most likely via the PXELinux boot menu):
append ... nbdroot=220.127.116.11:2000 nbdswap=18.104.22.168:9572
In the example above, 22.214.171.124 stands for the IP of the server providing NBD root, and 126.96.36.199 stands for the IP of the server providing NBD swap.
This overrides the results from ipconfig that usually drive the ltsp_nbd script responsible for root filesystem mount.