Firstly, a longline with an empty hook can-has-and will end up in your tail rotor especially while decending at high speeds if you fail to consider the following. The factors to consider are longline length, type of longline (Steel, spectra, rope, or other) and the weight of the hook. Some lines with multiple hook control wires can have major aerodynamic drag so be carefull.
I assume that when you are conducting external load operations you have a door off, perhaps this is the reason for the VNE restriction.
Are you sure your training pilot said "mast bending" and not MAST BUMPING?
Cheers,
FA