I’d really like this feature as well!
I’m wondering if this could work as an end-run around the immediate processing problem…
If you upload an image into a post… It will be watermarked.
Would it be possible to add a dialog to the tinyMCE image editor, so that after you inserted an image into a post (now watermarked) you could click on the image and open the image editor and have a button that would regenerate the image sizes from the (un-watermarked) full size image, effectively removing the previously applied watermarks for that image.