NetBeans GUI Builder allows to preview the designed form in other look and feels. Unfortunately, there is no official support for simultaneous usage of several look and feels. The current implementation is based on vulnerable DelegatingDefaults. The previews provided by this implementation don't have to be reliable.
It seems that it would be possible to implement the preview through sun.awt.AppContext. We should investigate this approach. If it turns out to be feasible to use this approach then the resulting previews would be significantly better and safer.