What I have done in the past is when shooting in super variable conditions is to have two cameras. One with lower ISO rated film and one with much faster film in it for those dark moments. Say the slower film rated at 400 and the faster at 1600 or 3200.

If the camera has modes, then either an shiftable P mode or like you say Shutter Priority. Then following the action with light, swap between the cameras depending on the amount of darkness there is.