I have been on many solo cruises. Going to Bermuda next month on Celebrity. When I go to the WJ, I also leave a sign "Occupied, coming right back" and I take an old paperback and leave it on the table. After getting a beverage and silverware, I go back for food and try to get whatever I want at once. After getting the food, I go back to see what the desserts are and I either get one or fruit. I try to get everything I want so I don't need to get up.
Tipping your chair forward also shows the table is occupied.