My .02 and this could be wrong.. Some hang a chain off the side stays and this might do something but from a technical standpoint, this likely isnt that effective. For a trailer sailer, I think a four gauge cable in the configuration described below would be more effective at reducing the odds of bad stuff happening.
First.. for the weight, something like a four gauge cable is going to have both lower resistance and inductance than a chain - and likely way better for both parameters. The four gauge wire makes a much better lighting conductor and therefore is also more likely to carry the current in the strike.
Second, the mast is the tallest, most conductive structure and therefore most likely to carry the current of the strike. So you really want the "down conductor" cable to be somehow attached to the mast. This might be just wrapping the cable around the mast or actually electrically connecting it to the mast. Lighting will just arc right though the cable insulation, in my opinion, the main thing is keeping the cable from getting blown off the mast. If you think its important, you can electrically connect the cable to the mast (grounded or not), I dont think it would matter much.
As the cable goes over the side of the boat, clip it to the stays. If the stays get energized, they will also easily spark gap jump to the cable.
If you have put some plate on the bottom of your boat to dissipate lightning to the water.. well.. hopefully it doesn't blow off or cause new problems with the trailer sailor hull that wasnt designed for the plate. And you of course alway see some size given for the plate for salt water but then its just left that the plate must be much larger for fresh water.. how much larger? Well.. way larger. Im just not going to bolt some plate to the bottom of my trailer sailor with a large wire running to it.. sounds like a good way to introduce new problems, maybe even affect resale value of the boat.
The later science on lighting and water is that the water surface gets charged as the lighting strike comes down and especially in fresh water, the lightning charge will want to go the water surface. So the four gauge cable that you attached to the mast and ran close to the shrouds just needs to end near the water surface. Maybe slightly above, maybe slightly below.. Im not sure it matters. What I think would happen is that when the charge came down the cable and got near the water surface, it would jump from the cable to the water surface regardless of what you have at the end of the cable. If you are worried about "grounding", either strip the insulation off the cable if you think grounding is better, leave it on if you dont want the boat grounded. My opinion.. it doesnt make much difference at all.. do what ever makes you feel safer.
Do the same thing on both sides of the boat. You would want this all to be very fast to set up because if lightning struck while you are setting this up.. it going to leave a mark...
If in addition you want to attach a jumper cable to the backstay.. why not. Just doing this alone however means you are relying on the backstay to compete with the mast for carrying the charge. The mast is both lower resistance and inductance and will usually win.
I used to sail around a lot of lightning (high altitude in Colorado) and worried about this a lot. I would do something along the lines of what is described above if I still had the sailboat in a high lightning area.
FYI, putting the end of the cable near the water surface so that it re-sparks and then the charge travels at the water surface is pretty much covered by patents by Dr. Ewen Thomson. So.. if you do this.. just make one for yourself.