keeping two axes synchronized while defining the feedrate of only one

I'm attempting to build a multicolor titanium anodizing machine it has one axis of movement and a variable power supply the goal is to be able to define the power supply as the x-axis and the movement as the z. That will keep everything synchronized and allow me to program my color gradients but it also means that given the same f value the actual z axis feed rate will vary depending on the rate i change the power supply, im looking for a way around this. Im not sure whats best, can i just remap a mcode and run a python script that can keep them synced up and use single axis z movements? how hard would it be to edit the base level linuxcnc code to change the way it calculates the axis feed rates? That would be ideal and im willing to figure it out but i dont even know where to start with that, any advise would be greatly appreciated. 

Assuming the power supply (X?) is controlled by LinuxCNC, shouldn't
this all be in the gcode with the feedrate just setting the total time?