With last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar you can have:
@startuml
skinparam rectangle {
roundCorner<<Concept>> 25
roundCorner<<Foo>> 45
backgroundColor<<Concept>> White
}
rectangle "Concept Model" <<Foo>> {
rectangle "e" <<Concept>> as e
rectangle "F"
}
@enduml
This has not been widely tested, so feedback welcome!