Pretty late with my reply, sorry for that. Maybe there has been already a formal decision?
I tend to follow Alan here for the regular flight shooting in the primitive classes, including warbows: only natural materials.
My home-spun linen strings for a 52" 50# bow wheigh c. 6.5 g, or 100 grains give or take a few gr, but those are close to the limit of my capacities (and break after a (few) dozen shots).
For broadhead classes synthetic string material is ok for me.