SK,

out of curiousity, where was AA asking for volunteers? At the gate (they usually don't assign gates until shortly before departure)?
At the check-in counter? In the lounge?

Here in the US, they normally make the announcement at the gate; however, at HNL they sometime start asking already at check-in. I've been on many full AA flights from LHR or MXP, but I've never heard any announcement requesting volunteers...