With last beta http://beta.plantuml.net/plantuml.jar
You can have:
@startuml
rectangle blahblah as bb #line.dotted {
rectangle "first3" as fst #77F;line.dashed {
rectangle third #red;line.dashed
(forth)
rectangle fifth #line.bold
}
rectangle "second" as scnd
fst -> scnd
}
@enduml
Is this what you are looking for ?