I am looking at doing a similiar itinerary ORD-HEL-BRU without any stopovers and all Finnair flight numbers. It seems fairly clear that AA's rules would apply on the outbound trip, but what rules apply when I check in with Finnair in BRU for the return to ORD? Like the OP, I would have 2 - 50 lb bags to check.

(I did a Star Alliance itinerary this year with similar circumstances and even though the flight numbers had the same airline but different operating carriers, when we connected to our LH flight, they made us pay an extra $60 each for the luggage before we could continue our journey. I wanted to avoid any surprises on this trip.)