We like the idea.
So with last beta http://beta.plantuml.net/plantuml.jar you can now have
@startuml
robust "Web Browser" as WB
concise "Web User" as WU
@0
WU is Idle
WB is Idle
@0:00:30
WU is Waiting : some note
WB is Processing : some other note
@0:01:30
WB is Waiting
@enduml
Date are more difficult to add : we will think about it.