It is a great idea. I'm having a hard time thinking of a reasonable direct-positive process that would work, though; the easiest thing might be to "solarise" (Sabat(t)ier effect) the egg during development, which sometimes produces a fairly normal-looking positive image (and sometimes extremely weird stuff instead). I like the fact that the curvature of the eggshell compensates for the curved field.
But why, O why, couldn't the author have photographed a chicken??