Over the years I have pressure tested a lot of chambers and coils as part of their certification. You need a bushing or a lug thick enough to thread into,and another one for a valve to let the air escape. Rubber plugs on the ends of tubes or the bottom bracket wont do with the kind of pressure you need to pop out a dent. You also can't assume the braze joints are leak tight.
A little bondo and touch up paint is the way I would go, this discussion is probably way off topic.
Jim McCoin
Fremont Ca.