After reading about flax in TBB4, I bought some raw flax "line" and backed a hardwood shoot with it (using fish glue).......and bent it until it broke. I was surprised to find that the flax did not keep down the "splinters" and failed at the same time and in the exact same places as the wood. I did the same experiment with sinew a while back and the sinew was mostly intact after the wood failed.
Conclusion: I don't think flax will help retain reflex or strengthen the back very much...unless the wood is very weak to begin with (like willow).
Heat treating, on the other hand, has definite possibilities. I got really good results on ash and hickory. I tried heat treating elm and the results were OK....but not as good as I expected.
IMO....I would forget the flax and concentrate on heat treating/tempering.