Originally Posted by
Vaern
In Terry Pratchett's Mort, Death is described as being able to pass through walls not because he is incorporeal or anything like that, but rather because he is simply more real than the world around him and thus solid objects yield to him as he wills it. Nothing is more real than Death, after all.
Anyway, if you want to throw a twist on things being more than 100% real, make the super-real illusions act as though real creatures and objects were quasi-real illusions. A 120% real summoned creatures deals full normal damage as a normal creature would, but if attacked by a regular character it only takes 80% damage because that character is less real than the illusion.