It's not possible in official release, but in last beta:
https://www.dropbox.com/s/koo42q3d9gxw288/plantuml.jar?dl=0
You can have:
@startuml
title this is %PLANTUML_VERSION%
alice -> bob : hello
!if %PLANTUML_VERSION% > 1201713
alice -> bob : A
!else
alice -> bob : B
!endif
@enduml
Unfortunately, this is not working with "old" release.
Thanks for the suggestion!