You do realise you can get anti-backlash gears?
Essentailly two gears mounted together and spring loaded out of alignment. You could do something similar using two gears mounted apart with a spring loaded belt between them, however you have to remember high acceleration forces could overcome the anti-backlash spring and allow backlash to develop briefly until the forces reduce again.
Then you don't get prefect motion with a straight cut rack, as you get a slight notchyness as each tooth engages/disengages with a resultant increase/ (there is a proper term for this, but it's essentially what makes straight cut gearboxes so noisy), but for a plasma cutter it's not really an issue. If you're really concerned, you could use a helical rack, but I wouldn't worry about it.