Ok. Nice idea!
In last beta
https://www.dropbox.com/s/koo42q3d9gxw288/plantuml.jar?dl=0
we've added a flag -pipeimageindex N in the command line, so you can now have:
java -jar plantuml.jar -pipeimageindex 1 -pipe < test.txt
(the index starts at 0).
Is this what you are expecting ?