text: The term "valid matter" recognizes that which is essential.  One may certainly go about playing priest and attempt to consecrate a can of cat food, but that is not the material Jesus chose.  I don't know why He chose bread and wine, but it is fact that He did.  It is the metaphysical validity that is being addressed.  Certainly this is for the Western Rite - the Eastern Rites have their own rules.  
  I would argue that you are allowing the physical reality of an illness to transcend the gift, trivializing what is clear in scripture in order to make accommodation that needn't exist, since the Cup is, clearly available.