It can be done.Back in the early 60s,when i kept discus,i forcefed several young discus,mainly wild caught browns and greens,that refused all food.Some success,and some miserable failures.
recently,i have a c.barronessa,going strong after5/6 months that i forcefed for the first 2 weeks.I used the syringe and tip from an old salfert testing kit.It was a very delicate operation,the food quantity was very small,and given several times over the day.
The food items were a mix of grated scallop/clam,with frozen rotifers.After several weeks,she began to take food from the water column.
forcefeeding is a standard pratice in herpetology,(reptiles,mainly snakes),to get young boas,and pythons to survive.Newborn mice,referred to as pinkies,are given to newborn constrictors,via a devise referred to as a pinkiepump.
Now,i know this is controversial,so,be aware that infections can occur.Forcefeeding a saltwaterfish is far different than forcefeeding a snake.Snakes can be given a food item,then feeding is not required for several more days.Saltwater fish need several feedings a day to remain healthy,so the labor and time are extensive,to say the least.