yes you can walk all the way down the terminal front at BCN and log all aircraft other than some on remote stands. But i think you will not be able to get back up there once you have collected your baggage. Other than that i cannot help. There is however a small museum down but at the far (domestic) end of the terminal which has a large collection of models etc and overlooks the area there most of the bizjets and smaller Iberia/Air Nostrum aircraft were parked (DHC-8, FK50, CRJ etc). Very vague but i hope it is of use to you!

Brgds

Ste