What sensors would be suitable for measuring masses of the range 0.5 g to 1g?
Weighing small masses is not easy, even more if you are restricted with respect to physical dimensions. You should look at how people who work in areas with similar requirements are handling such issues, for example, diamond traders or biologists.
Mettler-Toledo is an important supplier of scales for laboratories. Look at their laboratory balances. You'll see that even standard laboratory equipment is not good enough. And I can tell you that their high-precision balances will still not help you much unless you control the environment (no air circulation, constant moisture,...).
If you don't have to measure the weight of an object in rest, you could achieve better results, though. (Of course, the above equipment is not specifically tailored for your needs, so you could indeed build a better scale.)