Hey Charles, Not sure if you still need an answer. But i found a workaround. Funnily enough with a competitor software.Since they're all owned by the same megacompany I don't feel to bad. If you import all your polylines to Vectorworks they will automatically be hatches. Just add a fill color, and remove the outline, so it's just a fill. then export as DWG and import into Archicad. voila, you got yourself several tens of thousands of fills into Archicad. You can also try with rhino, but rhino takes a very long time to create hatches, compared to vectorworks. i tried with around 150'000 polylines that needed to be fills.
hope this helps,