Thanks Alan, that was really the case. However, that brings up another issue:
I actually used to instantiate the interpreter with
pyint = new PythonInterpreter(null, new PySystemState());
But, when I discovered that the code below fails to convert a Java object ("event" in this case), I, almost by trial&error found out that if I instantiate the interpreter with pyint = new PythonInterpreter(), the Py.java2py() method actually works.
PyObject pyClass = pyint.get("ClassName");
PyObject eventEntryPointCallable = pyClass.__getattr__("event_entry_point");
eventEntryPointCallable.__call__(new PyString(eventType), Py.java2py(event));
Oddly, the call to eventEntryPointCallable.__call__() silently fails without any error whatsoever. The Jython function (a static method in Python class) is just not called.
The docs on Py.java2py() say that it "Uses the PyObjectAdapter passed to PySystemState.initialize() to turn o into a PyObject." Well, I don't pass any PyObjectAdapter to the PySystemState. So, perhaps I have to set up the PySystemState instance a bit, so that the Py.java2py call works, but I have no idea where do I get a nice general-purpose PyObjectAdapter that will do the job. I know it must exist somewhere, because the event handler is nicely triggered, if the interpreter is instantiated with simple new PythonInterpreter().