Thanks for the info on the Estop, Hood. Does it work well on your other machines? I don't feel comfortable about merely cutting the power to the drives. They still have enough bus voltage to move a bit I think. I cut power to motors and switch in braking resistors. Depending on the resistors values the motor stop pretty much instantaneously.
Main reason I don't like large motors on small machines is I am afraid the motor would have enough power to damage the machine should something go wrong. But you're using a servo and can limit the current/torque to a value you like.
The correct way would be figuring the centre of gravity of the head and anchor it there, I think.
Regarding vibration dampening, this should not be a factor to be considered as the chain cannot transfer any substantial vibrations. The vibrations will be dampened by the head mass and also transferred to the column and base via the linear slides and thus it's important to have well preloaded linear blocks.