I did not eat here, but as I was buying park tickets in the am, at the Portofino, three different couples came and tried to see if they could get ressies. (begged!) Conceirge explained that reservations were not available the day of. I think he said there was some sort of no show list, but that was at the restaurant, and he could not help with that.
He said most people call the 800 # and set them up before they come, like when they book their room, months in advance.
If you are not staying on-site, then I still think you call Universal's 800#