Question

is it reasonable to pass a Drawable or Bitmap across via onRetainNonConfigurationInstance() to save the expense of calling res.getDrawable() again?

Answer 1

You should not pass drawables. Android may choose different drawables from resources after changing orientation.
In my project i am passing only data, which i loaded from internet.

http://developer.android.com/resources/articles/faster-screen-orientation-change.html

Here is written:

Be very careful with the object you pass through onRetainNonConfigurationChange(), though. If the object you pass is for some reason tied to the Activity/Context, you will leak all the views and resources of the activity. This means you should never pass a View, a Drawable, an Adapter, etc.

Sorry for my English =)