It's funny, I was going to start a thread about this exact topic yesterday, whether or not netbooting 68ks is possible.
I did see on eBay a NuBus card that claimed to be able to do this a while back and googled for references. It seemed to be true, if not a very well-known or popuar product, although there are many possible reasons for that. I didn't buy it, maybe because it was too expensive, maybe because it didn't include the software. I don't remember. I don't remember what it was called or who made it either, but I can't find it again.
The thing is, netbooting a computer means doing some somewhat complicated networking stuff - configuring, then downloading a kernel, then mounting a network file system of some description - before really loading the operating system. So it's not enough to just have a networking interfcae that the OS supports, it's got to have explicit support doing all that in firmware. I don't think an old Mac can do it with a 'standard' NuBus NIC.