Well, I never build a canoe - only paddles for them, but I think "they" had to use what was at hand AND somewhat waterproof. Whit emphazise (spelling?) on the later, right?
I used pine pitch often as "natural hot melt" for points and it worked okay. Though, with hard hits, points often get loose, because it really is brittle stuff and will break if flexed. I can't think of it as being usefull in any bow backing.
BTW: Nice bow there! If you "need" more FPS and already plan to do an other string (12 strands B-50 will be more then enough IMO) what about dropping brace high a bit, to get more power stroke?