Using this hack:
@startuml
skinparam shadowing false
skinparam rectangle {
BackgroundColor<<PLACEHOLDER>> transparent
BorderThickness<<PLACEHOLDER>> .1
BorderColor<<PLACEHOLDER>> transparent
FontColor<<PLACEHOLDER>> transparent
FontSize<<PLACEHOLDER>> 0
StereotypeFontColor<<PLACEHOLDER>> transparent
StereotypeFontSize<<PLACEHOLDER>> 0
Shadowing<<PLACEHOLDER>> false
}
rectangle a <<PLACEHOLDER>> {
}
class b
a -> b
@enduml
you may be able to do what you want, you may need to change some of the parameters though because they seem to not work by now.