This premise of this question is that a team is using 11-12 motors and is not allowed to employ pneumatics functionality, but wishes to use pneumatic components for other purposes.
Specifically, is it legal to use a pneumatic cylinder as a shock absorber provided pneumatic reservoirs are not mounted on the robot and no pre-charging is performed?
No, this would not be legal. <R11> states, for teams choosing to use more than 10 motors:
Pneumatic cylinders are considered pneumatic components, thus would not be legal for teams choosing Option 2.