Semi-circular rule for choosing a FL: Risk of head on collision with borderline headings (179, 359)?
The adoption of semi circular rules is not meant to prevent collisions on its own. Rather, it is one of several safety nets contributing to a safe and orderly flow of air traffic. In uncontrolled airspace, normal rules of the air apply, for example, two aircraft meeting head on must both alter their heading to the right in order to both pass with the other aircraft on the left.

The actual "risk of collision" you are asking about, for someone flying in uncontrolled airspace, is incredibly small. One reason is that the amount of traffic in uncontrolled airspace is typically quite low, since authorities will most likely establish air traffic control for congested areas. Even in uncontrolled airspace, flight information service is provided in order to - among other things - prevent collisions. And collision avoidance systems, such as TCAS, will also help. Crucially, all aircraft have windows, and the pilots are trained to look out in order to see potential conflicting aircraft and react in time.