Call Finnair. Then call AA and ask for oneworld support.
If they allow their top tier flyers to reserve exit row seats they must allow Emeralds to do it as well. Looking at Finnair's pages, they seem to indicate this:
http://www.finnair.com/INT/GB/flight...lect-your-seat
http://www.finnair.com/INT/GB/finnai...mbership-tiers
just becuase the webpage doesnt allow it doesnt mean it cant be done. Indeed, if AA told me it was a violation of the oneworld agreement, Id pay for it then charge it back.