Cool, thanks for the clarification. Indeed it's a good and practical idea for a small app. As other comments have said, (some) people might happily pay for this app.

I luckily won't need such feedback loop anymore, had some mild lower back pain show up over 10 years ago and bought a chair without a backrest that, after 3-4 weeks of struggling, trained me to sit up straight. Now I have some random cheap office chair with a backrest, but I rarely lean back to it. Funnily, I was going to give up using that "backrestless" chair after 2 weeks of inconvenience, but decided to give it one more week and then the magic happened :-) Mild lower back pain automatically gone.