I think OP was also effectively inquiring as to whether the it's the rules of the booking carrier or the operating carrier that apply. I don't think there's a generic answer applicable to all carriers/programs, and I think sometimes it gets complicated.
But I guess you're suggesting that a passenger on a FlyingBlue award on a DL flight will be able to pre-assign a seat. Beyond that, will DL status help here, since the DL FF# won't even be in the reservation (and not sure you can even put it there on a partner award ticket?) Or is FB status relevant for the FB award ticket?