That would be the hilarious thing ever to see. As the other armies flee from the mountain of muscle and teeth, a wave of mad docks, meks, and loota boyz are running right at it to try to turn it into a mount.
I would imagine a good way to balance it as we'll is for it to be made more "orky", as looted vehicles are. Maybe have them unstable, and more difficult to control. I would also think a cool idea would be for the Mek to have the ability to paint vehicles. After all, "da red wunz go fasta!"