Tweaking a Macaroni & Cheese Cake
The trick was in playing with the thickness and the application of the egg wash. On the second attempt, I kept the springform pan filles to only about 2-3 pasta in the layer (pushing awy from the loaf idea, more toward a flatbread).

After adding 2 1/2 cups mac 'n cheese to the springform (sprayed and dusted), I baked fifteen minutes at 425'f, pulled it, sprinkled herbs on top, poured 60 ml egg wash (1 egg: 1 T aq) mostly-evenly around, baked another fifteen minutes.

This resulted in a reliable flatbread crust for the grilled cheese sandwich that balances the quiche-y-ness with the pasta pretty well. Basically, the baked egg wash acts like a kind of cement. I followed up with another wash on bottom, flour, cornmeal and saute in canola on a cast iron skillet.