The rules were here before you joined APUG. It isn't put in place to neither exclude you or include you. Here's what it says:
Please refrain from posting any images that have been manipulated digitally such as converting color images to black and white...
It specifically excludes what you are proposing to do. How about this? Put your slide on a light table. Shoot it with B&W using a macro lens. Then scan THAT. Since the "manipulation" is done in analog fashion, and film scan is allowed, you will be able to post your image.
I have done this myself from B&W negative to B&W positive-negative. It is easy and works very well. I even have an image posted on gallery using this method.