Is there a way to prompt a user to +1 our Unity Android application from within the game itself?
This appears to be possible through a native Android application through the PlusOneButton:
http://developer.android.com/reference/com/google/android/gms/plus/PlusOneButton.html
pafla
2
You could create an Android Java plugin. That would of course require some knowledge of Android App development.
https://blog.nraboy.com/2014/06/creating-an-android-java-plugin-for-unity3d/
Also, there is an Asset for this (and other Google related stuff):