I had a couple different thoughts going when I replied, but when I reread your original post I realized that the tusks were already completed . . .
1. I would coat it with any base paint, acrylic is fine . . .
2. My thought about the webbing was in case you wanted to drill through it and add wire, since it is already cast . . .
3. I am not sure what process you used to get a solid plaster cast of your face; I usually do strips of cheese cloth and then smooth it with plaster . . . but depending on whether you do reverse casts you can always lay a layer of net or mesh behind to give the plaster more strength. I also, generally line the inside with felt (especially if it is worn on the face)' I spray glue the felt and shape into the features . . . this also strengthens it . . .
4. again, if your tusks are light weight, I base paint your mask, glue on tusks, base paint them, and then decorative paint the whole thing (I usually put numerous paint layers on so it looks rich and thick and doesn't chip as easily . . .
Anam Cara