I agree it would be very useful, but I think it will be cumbersome to implement for EYB and add quite a bit of complexity to indexing (which is very simple already... LOL :) : what is a substitution ingredient, where do you draw the line? Also, that would only be available for recipes going forward, how do you go about the existing gigantic stock of recipes?
Maybe EYB or the developers could come up with an easy way to implement it (for example categorising the ingredients, in which case you could search for vegetarian ingredients + chicken stock or whatever), or I don't know..
Or maybe use labels/bookmarks to say the same thing e.g. "substitute ingredient vegetarian"/vegan/etc.? (I know it may be cumbersome to assign them but just as much to index no?)