"If you can find a decent sized ( smallish) antenna that will fit the vent pipe, I would probably go ahead and try it. Something like a dual band comet or diamond mobile style, that is maybe 2 or 3 feet tall and fairly slender."
I have exactly what you are talking about. I have an Arrow 146/440 Ground Plane mounted on a short 4 foot mast that is hose clamped to my copper sewer vent pipe. It is a small well built antenna but that is what I wanted. I only needed something that I could use to hit the local repeaters so I stayed with a low profile installation. I'm still waiting on the wife to chew at me for putting up yet another antenna. Its been up since Dec. of 07 and she hasn't said anything to date.
It works great.