They will work AT curing syrup till they have to cluster. I dont know if you are close to target weight or if you are far short of it. If you have put on 4 gallons of syrup in less than a week for instance, it likely wont all get dried down. If they have been taking smaller amounts they wont have such a backlog. As mentioned insulate the top and feed warmed up syrup if you still need more weight on them.
If you have large amounts of watery syrup on board you should be extra careful to get sufficient ventilation in a way that does not cost too much lost heat.