You may want to automate some of this in your build, but this will work. It's how I got a splash screen to appear in Android while the app was loading (not after it had loaded or with an artificial delay)...
Create a file in platforms/android/res/values/styles.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="MyTheme" parent="@android:style/Theme.NoTitleBar">
<!-- set the splash screen as the background image on all windows -->
<item name="android:windowBackground">@drawable/screen</item>
</style>
</resources>
In your platforms/android/Manifest.xml replace the theme with @style/MyTheme
In your config.xml file add the following two lines
<!-- make the webview transparent -->
<preference name="backgroundColor" value="0x00000000" />
<!-- cordova will copy the splash screen file to screen.png,
but seems to ignore it after then
-->
<splash src="res/splash/splash.png"/>
Obviously you'll need a splash.png file to use.