...but have a plan B
I've done it with IB many times...however, a friendly note of caution...its not an official policy as in the US...completely up to whether the skipper allows it on the day. Madrid flight ops management have apparently been actively discouraging the practice and it has become more difficult than it used to be. I've found that flights through BCN and the other outstations are more relaxed. There is no paperwork associated with the IB procedure...usually just get to the gate and then ask the Captain. (Exceptions that I know of: In MAD report to the flt ops briefing desk to find the crew. In LHR report to the dispatch office located immediately behind the IB ticketing counter to receive a manual boarding card). I have usually been treated royally...including a day when I was desperate to get to work and the only seat left was a spare FA jumpseat...given to me with a smile and J class catering to balance on my knee! You will probably get some more feedback from the guys if you ask in the Spanish Forum.
PG