We always try to book a second Palo brunch once onboard. We go the second day and see if they have anything available. Why the second day? We're all in this together, and if demand is so high that all tables are reserved, it doesn't seem right for us to have two reservations if others will be left with zero (even if we are concierge, we still don't feel entitled to have priority for a second meal over others who want to eat there). But, after everyone who wants one has a shot on boarding day, we always check to see if they have spots available. We have been very lucky over the years with getting a second reservation!
So, hope for pixie dust, check with Palo once you are onboard, and keep checking throughout the cruise (there might be cancellations). Palo brunch is a not-so-secret jewel, we've been to Palo about 25 times, and savor each visit.