Path Constraint - follow on one axis

I have several helpers that are placed along a spline, I've set them to follow so that they all point in the 'right' direction. Trouble is, I'd like to zero out the x and y rotation values and just keep the Z rotation. This is easily done just by right clicking the relevant spinners at the bottom of Max, but I'm struggling to find a maxscript workaround for this.

Whenever I attempt to read the x and y rotation values it always returns 0, does the follow function effectively 'hide' the true values? I've also tried playing with the rotation controllers of the helpers to no avail, any ideas how I can achieve this?