Far as I know, no restrictions against trailers other than a speed limit. (55 mph when towing)
Police will not care what plate the trailer or the bike has as long as you have proof of ownership (registration or title or both) ... and proof of insurance.
Entering Mexico, Cent. America, S. America with a trailer should not cause any
problems either ... thousands of American tourists enter Mexico annually towing trailers of every shape and description ... but I've never towed a MC trailer there ... or anywhere.
The Bush Tec trailer does not look light weight to me ... and I'm pretty sure the
Canadian company, Trail Tail, near Kelowna, BC, is the better trailer for a motorcycle. A fellow DR650 rider I know towed this single wheeled trailer from N. Carolina to Seattle, Wash (see pic below) using a 38 HP Suzuki DR650.
Zero problems.
Surfboards on board? really? Many surf spots in Mexico and Cent. America that I've surfed at had boards for rent or sale. I lived and traveled there since early 1970's.
El Salvador now has well established surf industry, so there will be boards. Similar in a few places in Mexico as well like Puerto Escondido and others.
But I haven't visited those popular Mexican spots in years so can't give current info ...
I would contact California Surf expert Garnaro. He traveled all of Africa carrying a board on his DR650 Suzuki. He is also a Mexico and Cent. Am expert.
Fantastic ride report.
He would have more current info about availability of boards down South.
http://www.horizonsunlimited.com/hub...urfboard-72401
Cross country pulling trail tail.
Trail Tail Trailer Kelowna, Okanagan