We live locally and often book cruises "Last minute" - a week or two before departure. Because of this, I don't think we've ever been able to book Palo brunch online - we've always done it on the boat!
As Palo Brunch is one of our favourite meals on board, that is always our priority as soon as we get on the boat. We ask "where are bookings for Palo brunch being taken?" (depending on the boat it might be in one of the nightclubs or at one of the podiums at Palo or Remy) and we head straight there. Sometimes they'll be taking names right then, or sometimes they tell us to come back in an hour, and we wait around.
So far we have always been able to book on board (I think six times now?). Once they did not have a spot (we were a little late getting to the podium and had grabbed lunch first) but they took down our names, and called us later that day to let us know that they had squeezed us in =)
Another option - if you are in a Concierge level room - email or call your shoreside concierge castmember and they can probably find a spot for you (that's what they did for our upcoming cruise next month on the Dream).
Hope that helps!
Hope that helps!