Here is how it currently works:
when the library is initalized in the context of a stand-alone application, it creates a QGraphicsScene which is rendered in a QGraphicsView when the library is initalized in a Java context, it creates a QGraphicsScene which is rendered in a QImage at Java client request

In both cases, I create a QApplication but in the Java context, I don't call QApplication::exec().
The QGraphicsScene is modified by the time task of a QTimer.
In the Java context, a QMutex ensures that the time task and the Java client request to get the scene bitmap data don't collide.

It basically works but it also crashes randomly and here is my current investigation state.
First of all, I've replaced the QPixmap with QImage since the final QImage rendering is made outside a GUI thread (and Qt complained about that).

Now there is a single remaining warning at client startup:
"QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread"

Here is the code of the Java context rendering scheme:
@
QImage image (w, h, QImage::Format_ARGB32_Premultiplied);
QPainter painter(&image);
painter.setRenderHints(QPainter::Antialiasing | QPainter::TextAntialiasing | QPainter::SmoothPixmapTransform);
fScene->render(&painter);
painter.end();
@

Note that it doesn't crash when I comment out the 'render' call.

I don't get anymore warning from Qt and the developer documentation only talks about QWidget that should not be used in a non GUI thread. Most of the crashes occur when modifying a QGraphicsSceneBspTreeIndex (or QGraphicsSceneBspTreeIndexPrivate) object.
It looks like there are remaining unsafe operations called. Any help would be greatly appreciated.

[edit: fixed paragraphs -mariusg]