A projected capacitance touch screen like the type in your phone is quite capable of creating a reasonably detailed map of the areas of the screen that are covered by your fingers. You could definitely use this to infer the amount of pressure being applied by looking at the contact patch size, and indeed this is apparently how Apple's Force Touch on the watch works (note, the Macbook's new trackpad has actual force sensors on each corner). However, you can probably imagine a lot of issues with making this work reliably, as everyone's fingers are different sizes, and you don't always touch the screen with the same part of your finger.
Another thing is that you can tune the sensitivity of a touch screen so that it can detect objects nearer or further away (which is how glove mode works on some phones) but then you lose the ability to differentiate the actual contact patch of a finger. It may be that Apple is using a sort of analog sensing system where they can infer the contact patch and the part of your finger that is slightly above the surface by actually measuring the amount of projected capacitance rather than just apply a threshold. Unfortunately you would not be able to do anything like this without your own hardware.