QAbstractEventDispatcher::EventFilter's argument type

I'm trying to develop a public API using low-level event filtering with QAbstractEventDispatcher::EventFilter. While the type of the message argument isn't specified in the documentation it can seemingly be derived from the window-system used when reading the code.

API stability wise I'd really love to see some guarantee given to message's type on a per-ws base so I could happily static_cast it to its actual type. Is that possible or does it impose too much of a constraint on Qt's extensibility?