A feature request was open for this and is assigned to our developers:

Create a 9 patch splash using the draw9patch application found in the Android SDK (http://developer.android.com/tools/help/draw9patch.html).
Save it as *.9.png so that the device will recognize it as a 9 patch image.
Set it as the splash image in Unity in PlayerSettings for Android.
Build and test on device, see that the image isn't recognized as a 9 patch. This is because the images is renamed to "splash.png" so the device can't know it's a 9 patch image.

SUGGESTION:
Maybe add a checkbox in the PlayerSettings to tell if the splash is a 9 patch image or not.