Probably more info than anyone is asking for but I found something interesting while searching patents Trev, this taken from the Background of the Invention section from United States Patent #6430928 Iyer, et al. August 13, 2002:
"Whereas hydraulic clutch actuators, and particularly pre-filled clutch actuators, have enjoyed significant commercial success, they have the ability to transmit vibrations backwardly through the system from the clutch to the clutch pedal with the result that the operator may experience unpleasant vibrations at the clutch pedal.
Specifically, imbalances in the crank shaft of the vehicle engine or engine firing impulses are transmitted to the flywheel which undergoes a swashing movement, the flywheel swashing movement in turn leads to vibrations of the spring fingers of the clutch release mechanism, the vibrations of the spring fingers are transferred to the release bearing of the clutch, and the vibrations propagate backwardly through the hydraulic fluid in the slave cylinder, through the hydraulic fluid in the conduit interconnecting the slave cylinder and the master cylinder, through the hydraulic fluid in the master cylinder, and then through the master cylinder pushrod to the clutch pedal where they are experienced by the operator as vibrations of the clutch pedal. The propagated vibrations also generate a pedal growl which is audible to the operator as well as a clutch roar which may also be audible to the operator".
So that flat rubber piece is probably acting as a diaphram to dampen or attenuate any backwardly propagating vibrations in the system that could lead to pedal vibration. IF Iyer, et al. did their background/current state of the art homework correctly AND these possible vibrations apply to the NSX as well.