The backpackers backpack might be too heavy to carry on. I'm not really sure though. Before I carried on the guitar, I called Iberia and asked them, and they said they allow people to carry on guitars as long as they don't go over the weight limit. The guitar wasnt that heavy, but a a big travel backpack might be. I'm not sure what they weight limit is.
Also, the way I managed to carry on both the guitar and my smaller backpack, was by insisting at check in that the backpack was my "personal item" they allow, like a purse. I found them pretty flexible after i explained my situation, that i had been in spain living/studying, and was now moving back to the usa.
I'd call and ask, they can probably tell you.
Kelly