Civil versus public use is a non issue. Any agency that has a certificated aircraft most likley flies both civil ops. and PAOs. It is the mission, not the agency or the aircraft that determines if a particular flight is public or civil; as long as the aircraft has a non-restricted type certificate, like the EC 135. If they ever fly non-required crew members, dignitaries or anyone who would be designated a passenger they are conductiing civil operations and must adhere to at least part 91 maintenance guidelines. Any agency not doing so woild be foolish indeed if a savy laywer gets involved.