The projective texture mapping as written was based on CPU calculations. These are incompatible with GPU skinning.
A totally different approach would have to be taken… essentially you are starting over from scratch.
I think most of us off the top of our heads can’t even think how we’d do it.
Given the effect you are going for, I wonder if simply having a material with a second texture (and potentially second set of texture coordinates) will be better. After all it doesn’t seem like you are really doing projective texturing.