There would be no way to plagiarize, because the professor would have direct insight into each part of your writing process as he saw bits of your essay fall in

There would be no way to plagiarize, because the professor would have direct insight into each part of your writing process as he saw bits of your essay fall into place. I fear you give too much credit in that respect. This does not mean you could not lift passages out to integrate with your work without a professor being any wiser. It might make copying a complete text a little harder, but even there it wouldn't be hard to integrate some deliberate errors, let him catch and then revise. Finally, it would do nothing at all to stop you from hiring someone else to write it for you as long as you could either pay that other person to participate in the e-mail feedback or you were willing to do the revisions yourself.