Hi,
The goal of a RSS feed is to give to a RSS reader a list of the "latest things" so that when you add new things to that list, it will appear on the RSS reader and the user will be notified to check the new thing you added.
On a category page, there is a list of products, and thus a RSS feed makes sense there so that your users can be notified when new products are added to the store, or a specific category.
On a product page, there is no list of things, at least not by default. So I don't see what would be the goal of a RSS feed on a product page ?
So before letting someone subscribe to a product page, we would need to develop something, and even before that, we would need to identify something to be listed in such a feed. I'm afraid that what you're trying to do is not possible.