I agree about the flavoring making the host invalid. Is that correct or not, Alex?
Canon 924 makes it clear that for the Latin Rite, which covers all that the piece deals with, all that is involved as food is wheat (add water to make dough, flatten, and heat). The eastern rite does have slightly different standards on what is allowed—with the notable difference being yeast.
While there have been more precise definitions given, particularly of what constitutes wheat (much to Alex’s interest apparently), suffice it to say that messing around with this stuff is playing with fire, and literally trying to construct a Jesus that is more to one’s taste—perhaps an apt metaphor for those who have to rely on their own private judgment, and wittingly or not end up constructing a false god in their own image.