I see there's a promotion giving 1500 bonus Avios for bookings on Iberia.com at the moment, even if it's on BA metal.

If I book a flight on Iberia, can I then change the booking to BAEC, and if so would it appear in MMB for me to select seats (as GCH) as if I'd booked on BA.com?

Not really worth the hassle for 1500 bonus Avios unless it's straightforward to do!!