[Renoir]

No, what you describe is merely a torque couple which could only introduce a dynamic perturbation to what is already a mechanical resonant mast. It is the resonance which allows standing waves to cause vibration excursions at specific locations of the mast.

There are various modes of vibration and we are addressing only that mode which can exist in a fore-aft plane of your mast because you did not mention a standing wave problem in a lateral plane which could also coexist with a standing wave in the other plane.

I reiterate that if you have a first order standing wave that vibrates at a frequency which generates one wavelength corresponding to the length of the mast from head to deck (for example) then there will be zero excrusion nodes at the head, halfway down and at the deck. Any attachments to those points can do NOTHING to minimize the vibration in that mode. You can, therefore, play with using highmodulus line attaching the line doubled to a spot right at the maximum excursion of mechanical movement (say a quarter of the way up and/or 3/4 way up) and put tension on the line as far back as possible on the deck to gain a good angle. Check with others on this board for a good way to temporarily keep the line from slipping down the mast when you try this test to see if you can either dampen the standing wave or change its frequency along with a less objectionable motion.

Once you find at least a test solution come back to the riggers on this forum for advice on a real fix. Another test is to add a mass at one of those test points of a hundred pounds or so yet, obviously, that is not a practical solution.