Not sure, why you want to use Z vector instead of Y but:
-create vectorProduct node;
-connect your curve worldMatrix attribute to vectorProduct.matrix
-set first inVector to 0,0,1
-set operation to Vector Matrix Product
vectorProduct.output will return desired vector in world space
if you want to get this vector in camera space then you still can do this with nodes (probably it is better to script/code this part). You can use different methods for example dot product or matrix conversion. Lets use the second:
-create pointMatrixMult node
-connect vectorProduct.output to pointMatrixMult.inPoint
-connect your camera.worldInverceMatrix to pointMatrixMult.inMatrix
-now we can normalize this result vector using another vectorProduct node
-connect pointMatrixMult.output to second vectorProduct.input1
-connect camera.worldMatrix to second vectorProduct.matrix
-set operation to Vector Matrix Product
-turn on normalize vector
this one should work in theory, still I think it is better not to do it in graph editor.