Usually, I would use aforementioned to refer to a noun - the aforementioned song, book etc, but would it make sense to say "Aforementioned in the introduction..."? I'm writing an essay and I want to refer to a point I made in the intro in the next part, but I'm not sure the word makes sense in the context. Would it be better to say "As I stated in the introduction..."?
As you said, 'aforementioned' is usually a modifier in itself, and so doesn't take kindly to modifiers used on it. "Aforementioned in the introduction..." just doesn't make sense. It would be better to use 'aforementioned' as an adjective of the idea you are referring to, or, if you don't think your readers will remember that far back, do as you said and say "As stated in the introduction."