Assuming that simply enabling netboot on the mac server on your one subnet and doing nothing else isn't working (as this is often all you have to do...)
Setting netboot clients up on the same subnet as the netboot server in a 'mixed' network is in some ways trickier than setting them up on a routed network imho, as on a routed network you can just nominate the mac server as a DHCP helper on your router/layer 3 switch and this will make sure the netboot server will see the client requests.
This isn't really an issue with WDS being present on the same subnet. WDS uses some fairly simple PXE boot DHCP extensions but IIRC the apple systems are a bit more esoteric.
You may be able to point your clients to the netboot server with the following command in the terminal
sudo bless --netboot --server bsdp://ip.address.here