My guess based on attempting to assign seats on several reservations with mixed success:
1. The default state of an award reservation allows online seat selection at iberia.com
2. When you choose a seat, a human at Iberia needs to OK the assignment.
3. If that human is trained properly, your selection of an exit row seat will be changed to a non-exit row _and_ the reservation will be locked to prevent any further online seat selection. If that human is not diligent (about 75% of the time in my limited experience), your selection will be approved.
4. Once the reservation is locked, you can give up on getting a different seat.
5. If you select an exit row, that selection will persist until at least T-48 hours without any automated or manual changes by Iberia. Past that I don't know, because I canceled my reservations which had exit row seats. I suspect that these assignments would have held and that I would have gotten them at T-24 when checking in.