It definitely can and does happen. If ever the 'first impressions' rule takes effect, that's the situation it applies to. You've got about 10 seconds to win the favor of a GA in order to sit in comfort. That's a lot of favor to garner in that short an amount of time.
I generally consider myself charming (when I want to be
), even in the face of adversity (miss a connection, etc... - as it relates to flying) but it has only worked for me once that I got an upgrade just by being a nice guy. The stars really need to be in alignment for both the upgrade-hopeful and the GA.