I am by no means the first to think of this as a quick search will show but, after looking at the research paper of the FEA analysis in another thread it looked like the weight hanging off the Z rails would cause a lot of bending force to be exerted. Since EG is stronger in compression than all other directions I think it may be good to use a pulley attached to the top of the Z vertical and attached to the Spindle mount and a weight as counter balance riding on a track to counter this force.

It works much like an elevator counterweight so the only power needed for the Z axis is for de/acceleration and no lifting of the head. This is more of a side effect though of this system because this should transfer the force down through the vertical into the base. Since there is now much less twisting (around the x axis) force and only the torque from cutting acting on the rails (around the z axis). It also helps with backlash since the weight will keep the spindle pulled tight against the ball screw.

Other people have used springs or hydraulic dampeners but I think the counterweight is better because it keeps a constant force instead of providing more support at full extension.

These advantages would be great for most mills but I think especially for a EG mill this could offer additional advantages.