We really like the idea !
It's really a beta version, but with last beta
http://beta.plantuml.net/plantuml.jar you can now have :
@@@format svg
@startuml
Alice -> Bob
@enduml
or
@@@format png
@startuml
Alice -> Bob
@enduml
This way, you should be able to switch between png and svg. It's only working with -pipe.
Tell us if it's working for you. Right now, only png and svg are supported. Latter we could add more directive.