Good idea!
This is not possible with current release. However, with last beta http://beta.plantuml.net/plantuml.jar
you can have:
@startuml
skinparam belowForResponse true
Bob -> Alice : hello
Alice -> Bob : ok
@enduml
We don't like the name "belowForResponse" so we are open for a better name.
Is this what you were looking for ?