To my knowledge, Guest Relations will be open as soon as guests are allowed into the psrk, including PPO ADRs. Depending what time your ADR is and what time you are allowed into the park, you may have time to request a DAS if there aren't a lot of others doing the same.
as Lanejudy said the GR is open as soon as guest get in the park and stay open until all guest are out of the park with this said you many not have time to stop by depending on when they let you in the park.