Preparing slow cooked meats for the next day
I would suggest you follow your plan, but reserve and refrigerate the cooking liquor to reheat the meat in, which will help with restoring juiciness. 

However, be very careful when reheating and keeping meat warm. The meat should not be in the danger zone (5-60°C/40-140°F) for more than 2 hours, cumulatively, throughout its 'lifetime'. You should avoid moving the meat through the danger zone too often as well. This means you need to cool the meat quickly, reheat it quickly, and minimise the time it takes to get to the plate. If you've got a long drive to your picnic, or it's going to be a while before you eat, I'd consider serving it cold instead.