Force an image transform to images inserted into Redactor
I wrote a plugin called Retcon which is able to apply bulk image transforms to images in HTML/Redactor content:

{{ entry.body|retconTransform({ width : 1024 }) }}
{{ entry.body|retconTransform(myAwesomeTransform) }}


There's currently no way to tell Retcon to only transform images that hasn't already been transformed, though – so this might not be exactly what you're looking for.