Can one of your letters of recommendation come from a professor in a department unrelated to your field of study?
I can't think of how an excellent letter of recommendation from a professor in an unrelated field of study could hurt you. Perhaps if it was the only (excellent) letter of recommendation, maybe then. But you mentioned you have an excellent letter from your employer and can (I assume) get at least one more good letter from someone else, so I think you are in a good position.

You are right, his letter of recommendation would not be about your technical skills related to EE, but it would mention your characteristics, work ethics, teamwork ability in a very good light. Research writing course is also not that far away from EE. It shows you are more diverse than the stereotypical programmer or engineer. You worked with this specific professor for 2 semesters, he could give you an outstanding letter of recommendation. I think his letter will give you more than any single average/solid letter from other professors.