Choosing the iOS devices as the primary support platform is a no-brainer.  It is the most popular platform and the audio hardware in different models are common (iPhone has taken away the 3.5mm jack :(  ). On the other hand, Android and Windows will be much harder to support because of their huge hardware variation. The decision is to begin the development of the driver for these platforms after the sales of SonarPen has reached a certain level. That also means the hardware design of the stylus must compatible with all the platforms.  

A good example would be the iOS detects the existent of a 1k ohm resistor between the MIC and Ground line to tell if the earphone has a MIC on it. Similarly, a Samsung Android device needs the resistor to be 1.4k ohm to confirm a MIC on an earphone. So my stylus has a 1.5k ohm resistor on its circuit for broader compatibility. 

A little thing worth mention is all iOS devices sold in EU countries have a lower maximum earphone volume than the ones sold outside. This limitation forced the stylus driver must be able to self-calibrate instead of hardcoded to work at a certain volume. Drivers are needed to rewritten for different platforms, but the experience of creating a self-calibrate driver will help a lot during the development for Android and Windows devices.