I used hot glue for the spines. Hot glue is magic. It conducts light really well, just like light pipe or side-glow fiber optics. We'll create the shapes first, then glue them onto each NeoPixel.
Heat up your glue gun and get some wax paper or other non-stick surface. The backing from sticky-back vinyl stickers works great for this. Make a whole bunch of teardrop shapes in a variety of sizes out of the hot glue, and let them dry and cool completely.
While these are drying, take a utility knife and carefully cut a square hole over each NeoPixel through the silicone sleeve, so the pixels are fully exposed.
Arrange your glue spines so they flow from large to small, and place them upright inside the hole on the NeoPixel strip. Use some more hot glue to glue them in place.
One cool feature of hot glue is that if you make a mistake, you can use 99% alcohol at the joint and the glue will become immediately and completely un-stuck. It's like an undo button! If you get your spines in the wrong order, or are unhappy with one, it's easy to take it off and replace it just by pouring some alcohol over the pixel. (Be sure not to do this while the lights are turned on, though!)
Now it's time to fit your strip into your dragon. For the areas where the strip goes along the mounting board, cut notches along the dragon's back for each of the spines to poke through. Take your time with this, and get it right. Judicious use of a heat gun can help if your spines are being unruly, but don't get them too hot or they'll start to droop. You can also trim them with scissors or a knife so they come to a sharp point at the top.
I think imperfection is actually on our side here. I love the individuality of each dragon spine.
For the areas where the strip is on top of the dragon, you can lay the strip on the paper mache and use masking tape between the pixels to secure it. I wanted it to look like the spines were coming right out of the back of my dragon's head, so about half my pixels are inside the paper mache and half are on top.
Add tape until you have a smooth neckline that doesn't give away the shape of the pixel strip.