From what I gather from the schematic and description.. The bike tire tube is filled with propane, which expands and pressurizes the "liquid" in the tank? I may be totally off but if thats how it's supposed to work I would put another ball valve or regulator between the blowgun and tank. I think that most blowguns are stamped for 90PSI but I have no clue as to when they explode, but I'm sure they will leak long before 90PSI. If I were you I would fill safer doing away with the tube and just using compressed air, what happens if the tube bursts or separates from the valve? Better yet I would design something with gravity feed for the fuel.
I'm also curious as to what your planning on using as fuel. Also make sure the hole that the fuel sprays out of isn't so small that it vaporizes the fuel. I used a plastic C&H blowgun with a cloth pilot flame on my "torch" and it gave good results. Also what pressure are you planning on taking this thing to? I would try and get the main hose crimped by a mechanics shop instead of using hose clamps, a pressurized hose spewing flamming fuel in all directions isnt a good thing.
As demilus said blowguns do leak (mine did) and so you might want to think about a longer extension tube for the blowgun as to keep the pilot flame and the main flame away from your hands just incase you do get some fuel on them. Oh and trust me if it works like mine did the heat from the flame at 2ft away is very warm on the face, the farther away the better.
Long winded but maybe it will save a life and some 3rd degree burns.
BTW is it against forum rules to talk about flame throwers?
I don't wanna hijack your thread but here are a few pic's of my setup, maybe they will be of some use to you.
Blowgun/Sprayer
Tank
More pic's and video if you would like to see.