I'm stuck! I have already set up a particular effect following a null path in 3D space. I have decided that I now want a word to follow that same path shape bending along its curves, but of course Text only seems to want to follow a Mask path. I don't want to make a Mask path and try to line it up with the existing Null path. So, is it possible to convert the Null path with its Bezier curves into a Mask path? Or can you suggest another way to do this to achieve a precision line up.
Of course, I don't want to start from scratch by the way!
You can copy/paste the position keyframes of the null to a mask on the text layer but it will become a flat 2D path, dropping the Z position data and probably shifting the points around in the process. I don't think there's any way to do what you are wanting to do inside of AE because the text on a path feature only works with 2D paths. You can fake it somewhat by rotating the text layer itself in 3D space and then changing the orientation of the characters so they appear upright. That would let you move in the Z axis but it would still be restricted to movement along a 2D plane.
The only way I can think of to do what you are talking about is to break up the word into individual characters and copy/paste the 3D null position keyframes to each character layer's position property and then stagger them in time to acheive the correct spacing between the characters in the word. You can turn on layer>transform>auto-orient>orient along path and see if that works but I think you'll find it is a little twitchy and you may end up having to orient by hand to get it to behave the way you want.
Maybe someone else will have a better/easier solution.