The headstock-weight thing has been around for a while. Some folks here may be familiar with the Fathead that the late Aspen Pittman produced. This was a headstock-shaped brass plate that fit between the tuners and headstock to increase mass. And if memory serves, there was another product, also brass, that clamped to the headstock.
Generally, when you pay a decent price for a well-made guitar that uses good materials, attention will be paid to the match between neck and body, and choice of wood in achieving balance. The body shouldn't damp the neck and the neck shouldn't damp the body.
A good quick test to see if increasing headstock mass can improve sustain is to press the guitar headstock against a door jamb (while standing, obviously) and see if the mechanical coupling of the guitar with the mass of the door jamb makes a noticeable difference in sustain as you play. It's not foolproof, but it IS cheap and easy. Just know that I do not recommend attaching a door jamb to your guitar, no matter how much you think it improves things!