Two extra things emerged from reports. The plane had called an engine failure before the incident. And another Liberty pilot on the ground refuelling saw the plane heading for the helicopter and called on the RT to warn the heli.
These VFR corridors really are a problem. This is exactly the same type of incident that could happen in, for instance, the Manchester Low Level. That location is made worse by ultralight/flexwing traffic not showing up on radar and the very low 1250 QNH alt.
I've heard there is a plan being looked into - some sort of vertical separation applied to north & southbound traffic.