Hi John, You're right that this sort of thing isn't commonly available. There used to be a figure 8 strap that went from your upper back around the front of your shoulders with a little gadget in the middle of your back that chirped if you sagged. There are braces out there that are supposed to hold you straighter, but who wants to be dependent on a brace. There's nothing like it in my very thick rehab supplies catalogue.

If your gadget is a real taskmaster, you may find that your "straightening" muscles are exhausted from all the unaccustomed work of reacting to the buzzer. Good posture is a very complex thing. You have to lengthen the back of your neck without over-stretching part of your neck, bring your head back over your shoulders without looking up or down, bring your shoulder blades back to where they should be (closer to your spine and flattened down on your rib cage but not elevated up towards your ears or rotated), straighten your upper back without over-arching your lower back, decompress your rib cage ie. stretch the ribs apart back to where they should be, and correct whatever is going on with your low back and legs.

Some of our muscles are tighteners and some are weakeners as we get older. (google Dr. Vladimir Janda). If you work the weakeners too hard without stretching out the tighteners, it doesn't work as well.

I think your gadget is a great idea, and if you "invent a better mousetrap, the world will beat a path to your door". I'd sure recommend it to the people I work with, but I'd make sure they knew exactly what to do when it buzzed. A good physiotherapist should be able to set up a program to help you take full advantage of your excellent invention. You just may be on the track to something!