Sounds like an interesting project! There are several devices intended to solve the same use case which are currently available, although they work in slightly different ways. They are almost all a form of pressure sensor.
Probably the best known all in one commercially available is WiThings Sleep. It is a long pad which goes between the mattress and box spring. Because it is detecting changes in the person’s movements, not just weight on a pressure sensor, it can track a lot of different kinds of information, including sleep cycles, heart rate, and detect snoring. It has been clinically tested at a hospital. It typically costs around $130, although it’s occasionally on sale. It has Ifttt integration, which is how people bring it into smartthings. You put it on one side of the bed or the other, so if two people sleep in the bed, you can get two of them and track them independently. It’s been out for at least five years and seems to be pretty popular, it’s got a rating of just over four stars at Amazon.
Withings Sleep offers sleep cycles (deep, light & REM), snore detection, breathing disturbances and heart rate. Enjoy easy one-time setup and automatic sync to the Health Mate app via Wi-Fi. Also features IFTTT integration for scenarios such as dimming lights when you go to bed, or turning up your thermostat when you get up. You can place the sleep sensor directly under the mattress (between the mattress and the bed platform or box spring). Withings Sleep has been tested with a mattress thickness of 4 to 15 inches.
https://www.amazon.com/Withings-Nokia-Sleep-Temperature-Compatible/dp/B078Z1B34S/
I would think that would be your primary competition unless you can bring it in at a much lower cost. I don’t see how bedpost sensors are going to provide the same breadth of features. 
(By the way, at one point, smartthings had a similar bed sensor in development, and it got as far as beta test, but was never released to market.)
Your other competition will likely be DIY pressure mats. These are easy to make, and there have been a number of past project reports about them. See the smart chair project report for the most detailed discussion. Of course any discussion of Groovy code will be out of date and would need to be replaced with something compatible with the current architecture, but the hardware and general approach would still be the same.
So I think there’s a potential market, depending on price and features, but I’m not sure it’s really a new category.