Currently all class loading in HV happens vi the LoadClass PrivilegedAction. In the implementation we first check the thread context class loader and then the current class loader (unless we are loading own package classes in which case we use the current class loader). Historically that was a good approach, but in order to allow for customization in all sorts of environments it makes sense to have a way to configure the class loading approach. This could be done via a ClassLoaderService which can for example be set via the Configuration.
Depending on which way Bean Validation goes this could be a HV specific feature or something defined by the spec.