what's the thickness of this foam? I've used 1/2" eva before and it turned out to be a bit heavy when i used it for boots to cover up my stilts. I also think the 1/2" might but too much for covering with worbla.
you could also run EL wire down the entire spine through a rubber or vynel tube, and build the "vertibre" around the tube. then just house the driver under the back of the neck cavity here and you'd be able to change out the wire easily if needed. also may be able to use some of that fiberoptic stuff that emits light the entire length not just the ends