MSDN: https://msdn.microsoft.com/en-us/library/windows/desktop/hh454916(v=vs.85).aspx
MSDN (Pointer Device Input Stack): https://msdn.microsoft.com/en-us/library/windows/desktop/hh802883(v=vs.85).aspx
It seems to be related to Windows Ink in some way because if I disable Windows Ink from Wacom's settings it does not send these events at all. It also does not work in Wacom's mouse mode.
When the window is out of focus and you press and hold inside the window using the tablet pen, it does not show the annoying nagging circle. It also sends the pressure data properly (unlike with wintab where pressure doesn't work with the first stroke).
Support can be added by filtering Windows messages and injecting QTabletEvent, just like how the current WinTab forked from Qt is working.