I would assume so since the reservation system will be up and running. But you probably have to use the reservation page to select a specific reservation since your account page won't be working.
This is what I currently see for a P+C reservation (trying to get those in before the closure):
Due to the special conditions associated with this reservation, you may view or cancel, but not make changes to this reservation within this website. If you are trying to modify a Gold Passport reservation, please call 1-800-30-HYATT.
And you won't be able to reuse those points for quite a bit if you cancel....