Right now your only option is to image on an isolated network away from your production network. I suspect that WDS is using proxydhcp which will override your setting in dhcp options 66 and 67. There is no way around this AFAIK.
It would be interesting to see what WDS is actually doing this tutorial tells you how to do this with the fog server: https://forums.fogproject.org/topic/9673/when-dhcp-pxe-booting-process-goes-bad-and-you-have-no-clue
Or you can use wireshark with the capture filters of port 67 or port 68 or port 69 or port 4011
If you want us (or me) to look at it upload the pcap to a google drive and either post the link here or IM me the link and I’ll review it. It would be interesting to know exactly what WDS is doing here. But in the end, having an isolated (but routable) imaging network is probably your only solution. You just need a network where you can limit the broadcast domain to only that subnet.
