I used to be an air observer inspecting fuel mains for about 5 years in England. We always had an active transponder with a dedicated pipeline squawk, high visibility paintwork/rotor marking and pulsed landing lights. We have had several near misses ( including filed Airproxes). We used the current (1999>2005) notification system to let the military know where and when. The 'interactions' still occurred. We had TCAS fitted to the 206 which was a massive aid, especially across the low level corridors heading into the ranges etc.
I believe there was a fatal between 2 fast jets and a pipeline aircraft somewhere 'oop north before my time.