Thanks for everyone's input.
I like to tinker and it's fun to get your thoughts.
I'm not planning on making this for anyone other than myself, so anyone who wants to play with this idea is welcome to it at their own risk. I do wonder about how well the PVC will hold up over time. UV light may degrade the PVC - I could paint it. PVC is also brittle under extreme cold. I had a small piece schedule 40 in my shop. I'm not sure how available 1 1/2" schedule 80 is, but it would be stronger. It would probably be tougher to mold with heat.
I decided to use a simple dry bag rather than my homemade harness. It looked cleaner, but I am concerned about the bag rubbing on my top tube - both that it'll wear the paint on my bike and possibly wear the dry bag. My bike is aluminum so I think it wouldwouldb't be too big a deal - wearing a hole in the dry bag would be a bummer. I certainly wouldn't want to put too much weight in it, or extend it out too far.
I thought about issues with a marketable product. Using something other than PVC would make sense. Carbonfiber or another strong molded material would be awesome. There may be isses with too many variations between differing stem and handlebar combinations. I'd also assume you couldn't do this with a CF stem or bar.