Tip stalling
As you say, on the upper surface of an unswept wing, spanwise flow is towards the root.
Consider a wing swept through 90 degrees - the wing root is at the front, and the tip is at the back. The airflow on both upper and lower surfaces will be from root to tip, just by virtue of the speed of the aircraft through the air.
For real world values of sweep, spanwise flow from root to tip is airspeed times sin(sweep angle). That's not quite accurate because spanwise flow also depends on wing tip effects, but it's been good enough for the exams I've sat!
HTH,
O8