This patch fixes issues with the brush outline not updating position properly so that it is at least similar to the way it works with the regular mouse. There are still issues with the brush outline lagging, even with the mouse, but at least it works.
EDIT: That lag might actually just be specific to the OpenGL3 branch I was working on. Your mileage may vary.
Unsure if this fix will have consequences for non-Wacom tablets on OSX, but the main problems appear to be that the eventEater is setup to eat any mouse events that are synthesized by Qt (and it does appear eatSyntheticEvents is active). Unfortunately, any of these mouse move events from the tablet after a brush stroke are considered synthesized by Qt. The other issue is that after the end of the stroke, the call to ignore cursor events is not properly disabled in the Leave event, but appears to work when deactivating ignore cursor events in TabletRelease.