Hi, Patrick,
I agree with Sil that with only six days for walking, your best bet is to walk a continuous stretch for those days, otherwise you will waste your days on buses.
For me, there are several beautiful 6 day stretches:
Irun to Bilbao
Comillas to Villaviciosa
Aviles to Ribadesella
Mondonedo to Arzua
Lugo is actually on the Camino Primitivo, and I haven't walked that route, but I hear it's beautiful. And it would give you the 100 km necessary for the compostela, as Sil notes. It's a bit harder to identify an accessible starting point on the Norte that's the equivalent of Sarria (that is, 100 km out). The closest seems to be about Baamonde, but that's not exactly a hub.
A totally different possibility would be either the Camino Ingles from Ferrol or the Portugues from Tui (or a little further back in Portugal) , both of which could be done in that short amount of time.
You've got lots of choices, so have fun planning. Laurie