UML activity diagrams can be more complex than currently supported by either old- or new-style PlantUML activity diagrams.
In the example below, I managed to almost get what I want by leveraging PlantUML state diagram, which notably support entrypoints and exitpoints that can be used in place of InputPins and OutputPins (respectively).
However, for the illusion to fully work, the entrypoints & exitpoints should be empty squares instead of circles (without the X inside exitpoints).
This square shape could be associated to <<inputPin>> and <<outputPin>> stereotypes, for instance.
There should also be expansion nodes (e.g., with <<expansionInput>> and <<expansionOutput>> stereotypes), shaped as four squares stuck together to form a 4 by 1 rectangle.
Note that InputPins and OutputPins have the same shapes. Similarly, ExpansionNodes used as input and output also have the same shapes. It is however convenient to keep separate stereotypes (e.g., <<inputPin>> and <<outputPin>>) to control their placement.