Yes you can, but many others have the same idea, especially close to fireworks time so if the lounge is full the hosts/hostesses on the second floor will not let anyone up without a reservation. You can go earlier in the evening and show your receipt to return to view the fireworks later that evening.
Personally I think the fireworks from CG are overrated. Go to MK and watch them where they are meant to be watch from. Have some drinks at the Poly before hand.
We went to Cali Grill last year and the meal and view of the fireworks was AMAZING. This year, we're heading to the bar at Cali Grill early for cocktails and using our receipt to go up to watch the fireworks later that night. From what I've read on here, if you go early (I think it opens at 5), there should be plenty of room in the bar.
We were there 2 years ago while the Christmas party was going on and the music was still piped in. Can't comment if it's changed or not since then though.