Use two bundt cakes. Carve, frost, and place battery LED's on the inside.
Chill the two bundt cakes separately overnight.
As Matt suggested, you carve out the middle. I would hollow out more cake from where the face will be. I would then plaster it well with melted chocolate on the inside so it will be fairly rigid.
I would brush more chocolate on the thick parts where the eyes and mouth have been carved out, just to make it as stable as possible. Chill again before adding fondant.
In theory I suppose you could hollow out a chiled cake, on the inside, but just enough to insert one of those plastic christmas ornaments, that you can get to fill and decorate in craft shops. Usually you can get those in different sizes, plop LEDs in that and insert into cake. When you then out the second cake on that, the ornament should be stable enough to hold up the cake, sort of like an inside (round) dowel and you won't have to worry about the cake collapsing on itself
One of my friends did a pumpkin cake that had a cake in the back half and the front made out of gumpaste. The front was basically a shell, if I remember correctly, so she could put lights inside and it looked like a real carved-out pumpkin. Best pumpkin cake I've ever seen http://www.welldressedcakes.com/index2.php?v=v1 It's in the carved cake gallery
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