[intellec7]

I am not sure if this could work, but what if the PID input were time? If the slope of the robot in relation to the line is small, it will take longer for an error to occur. (one of the line detectors to detect the line). If the difference in slope is great, then an error will occur more quickly.
Perhaps pseudo code would be something like

:Beginning
Begin Timer
While line not detected
Go forward
Stop Timer
correction = PID(timer,sensor)
GOTO Beginning

Just a thought.