We've just published a new beta https://www.dropbox.com/s/koo42q3d9gxw288/plantuml.jar?dl=0
This beta should better handle the -pipe option : you can now send several diagrams in stdin (please use appropriate @startXXX / @endXXX delimitor for those diagrams).
Right now, there are no delimitor between pictures in stdout : you have to detect PNG header start : is it possible for you ?
In the future, we may add some optional delimitor (using for example a -delimitor flag) in the command line.
This has not been widely tested, so please feedback if you find issues!
PS: Note that you could also use the FTP option to generate several diagrams without having to restart java process. See http://plantuml.sourceforge.net/qa/?qa=5694/c%23-plantuml-jar-alternative