This probably isn't much use, but I will post it anyway.
http://www.bushcraftuk.com/community/showthread.php?t=3838
As I understand it, after being strung for a bit, animal fibre strings stretch as far as they will, then no more, so that isn't as big a problem as I used to think it would be. More of a problem, certaily with rawhide, is the thickness of the string. I think that indian arrows meant for use with such strings were held to the string with a pinch grip and didn't use deep nocks.
I learned a lot about cordage making from the Bowyers Bible, Vol 2. It shows flax being spun into single ply cord using a rather neat lash up and a power drill
It also discussed the advantages of multiply cord and the strengths of different materials.
How strong is the bow you need to string? Usually modern strings have quite a big safety margin, primitive strings will have a lot less. If it was me, I would probably do a lot of testing and practice of cordage making before I trusted any of my bows with any of my nettle cord
In the mean time I would happily make a string from cobblers linnen thread heavily waxed with bees wax.