Regardless of its legality (IMHO I think it would be legal, and very cool) I think your ideal solution would be to use something like this
electronic flow control unit. You could reduce the flow of air into the piston as it reaches the end of its travel.
Alternatively, if you relied on gravity to bring the arm down rather than power it (more efficient on air usage) you you program a duty cycle into the "lift" solenoid valve, i.e. when the arm is initially lifting the solenoid valve is open 100% of the time, but as your arm nears the top of it's travel the solenoid would proportionally reduce its duty cycle as appropriate.
Using a fixed flow control valve will penalize you in overall ground-to-lifted time (if you will) but would work just fine. My second idea doesn't cost you any weight either, which is always good

Just throwing a few more ideas at you!