IMHO done right it will look fine and I don't think it will weaken the frame enough that it is going to fold up on you. That being said, the bikes I have without brazed on cage mounts are nothing special and mostly hi-tensile and I still don't want to do this. Velo-Orange and other places sell better options for clamping the cages to the tubes.
__________________
"It is the unknown around the corner that turns my wheels." -- Heinz Stücke