Hi!
Containers are a powerfull tool. Unfortunately it's currently impossible to assign external referenced materials to objects inside a container. Containers don't allow the use of X-Ref materials and other options to reference external materials are currently not present in 3dsmax. So, please make an option that allows the use external referenced materials for objects in containers. Remove the X-Ref materials restriction or invent a new solution, please.
Thank you and greetings!