Bpbatch is a piece of software I found at the University of Geneva. It was developed there to help control the workstations on the network. Later, a company called Rembo was formed to commercialise it. Rembo has since been bought over by IBM.

Bpbatch does not seem to have ever been FOSS, but it was freely downloadable as a binary.

It is not the only solution to the network boot problem, but it was certainly the most flexible that I came across.