Yes, this works! 
Caveat is that a single face wont map from 0-1 (or maybe would if the uvs were changed, im not sure) but it doesn’t crossover in the middle and gives a linear value I can use (in the playground above its like 0.4-0.6 or so on the front face, continuing wrapping around the cube).
Here it is for posterity using a plane (which does map from 0 to1). Might be good for graph-type things. Or 3D darts 
Thanks again!