Gluten, MSG and aged cheeses all give me migraine headaches. Is there any substitute for aged Parmesan that might be safe for me?
No, there is no such substitute, sorry. The flavor of parmesan is dominated by both the tyrosine and the MSG it contains. If these two are triggering migraine, then there is no alternative which will be even close to the original in taste. Everything one can suggest as a substitute (marmite, certain fungi) is high in MSG.
You can of course use "substitutes" in a recipe just to provide bulk or a texture contrast, without attempting to match the taste at all. What you choose in such a situation will depend on the recipe itself and your personal preference for taste combination. It will be more like giving a completely new twist to the recipe than trying to imitate the original. We cannot give you a blanket recommendation on "what will work" for that case, because there is no such single thing.