I'm glad you've figured something out! Creating a native module makes a lot of sense for this very Android-specific behavior.
@spwoodcock you may want to consider this kind of concept to address your need to have an external web app set an entity in your form. Basically you could have your web app launch Collect and remember the last entityid that was selected, then have the first question in the form that's launched call to your app and request the last selected entityid. It's a bit silly but could be a fine stopgap measure while we define a forward-looking API that allows for setting the entityid directly.
@mgonzalez does your React Native app have a frontend? In an ideal world, could you use it like @spwoodcock wants to launch a form and set some values directly?
I'd be very interested in learning more about what you're up to. Consider writing an introduction when you have a moment. 