group {
  next-server 10.0.0.1;
  filename "pxelinux.0";
}

sin especificar la MAC para ver si cualquier sistema arranca pero no es así. Y lo que se puede leer en el propio “man dhcpd.conf” parece corroborar mi idea:

Imagine  that  you  have  a site with a lot of NCD X-Terminals.   These
terminals come in a variety of models, and you want to specify the boot
files for each model.   One way to do this would be to have host decla-
rations for each server and group them by model:

group {
  filename "Xncd19r";
  next-server ncd-booter;

  host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
  host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
  host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
}

group {
  filename "Xncd19c";
  next-server ncd-booter;

  host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
  host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
}