Though not an expert at this, I am aware of the "mast bending" issue. Mast bending is evaluated for drag inducing modifications to helicopters, I'm sure a long line qualifies.
All helicopters have masts. During a perfect hover, masts are loaded in tension and torque only, because the rotor is producing lift and torque only. Once the helicopter flies in a direction, the rotor is also producing thrust. The mast is then being "bent" to also produce thrust.
During original design approval, the mast strength is evaluated to assure that it will withstand all of the loads applied to it. This evaluation will be one of the considerations in determining the Vne. If the drag of the total helicopter is later increased by modification, the thrust must be increased to overcome the increased drag. This means that to fly at the original Vne, with more drag that originally considered, will result in more bending of the mast than originally considered by the manufacturer. In the absence of a complete re-evaluation of the strength of the mast (very costly engineering), modifiers will probably elect to accept a reduced Vne with the modification to assure that no additional bending loads are imposed on the mast.
There is a simplified flight test procedure to establish an appropriate Vne, which is conducted during the design approval program.
All of this is completely distinct from mast bumping.
So, if a pilot exceeds a reduced Vne with an external mod or load, they are exploring a previously unexplored realm in that helicopter.
I hope that helps,
Pilot DAR