Yea seems a little risky, but hell, I priced out a tube replacement on one of my bikes and it was around $100. Furthermore, they couldn't match the rest of my Ishiwata 019 tubeset, AND, it required a repaint.
If it's not a terribly expensive frame, this sounds like a good plan. How do you keep the water isolated in that one tube? What are you using to plug them up with?
i used a tube patch and rubber cement. haha working quite nicely. the frame is a jamis sputnik