Magnetic slide of Z-axis

[Constantine]

yes, it would be not a problem, but we would loose the main advantage - Simplicity:)

Any way lets say that we use at least 10kg*cm motors and screws of about 4mm pitch - that means a reduction from 2*Pi*1cm to 0.4cm = 15 - which means for a perfect screw you'd have a 150kg moment of force, those 2kg required for friction will simply disappear.

I'd place roller and magnet only where you need taking off and placing back the carriages.
Oh - by the way my Mark 1 machine was MDF based router. I've made them like that: