People are confused about how to update the splash screens during their Android build. Why not place all the default app helloCordova icons and splashes in the location you recommend, in /helloCordova/android/res/* instead of in /helloCordova/platforms/android/res/*? Then fill the config.xml with the lines needed to port the files over to Android via CLI as you recommend.
This example of proper placement and config.xml usage will remove all users' doubts about where to put the icons and splashes.
I assume you could programmatically set up the default app so that the entire /platforms/android/res folder structure is reproduced in /helloCordova/android/res/ to make it easy to understand where to put the user's own files. They'll replace those icon.pngs and screen.pngs with their own. You would program the same thing with iOS files in /hellocordova/ios/res/ .
Just a suggestion!