Hi Katrina. The negation symbol you have described is one of several symbols used in logic to denote negation. Since this particular print sign is not listed in the Nemeth Code, you can replace it with any of the other negation symbols as long as you explain in a transcriber's note, and as long as the symbol you choose is not also used in print in this context. I have attached an image from Wikipedia that shows other options. The tilde would be my first choice. "Np" would be my second choice.
Please let me know if you need further advice.