You've been able to make reservations online at Universal for years. It used to be through OpenTable which was 6 months in advance but a couple of years ago they changed systems to NexTable which is only a month in advance. Antojitos was only 2 weeks in advance but I'm not sure if it still is on the new website & Cowfish you have to book through their app on the day once you're at the parks anyway.
I doubt calling the restaurant directly will make any difference though because the booking system is the booking system but you could try, I guess.