It's likely expensive to repo out of country, so if you were a deadbeat and didn't pay the note, you could roll the rig into Mexico and tell BoA to come and get it. Sounds like so-called border letter is more to let BoA know the rig is out of the country, and and was said to be be sure insurance will cover it while it's there.
If it's a law, it's there to protect the lender, not you, so there is nobody to "catch" you in Mexico. Sure BoA could call the loan if they found out, but likely wouldn't, and even if they did, likely couldn't get to the rig to repo it anyway.
That said I would play by the rules and get the letter if I needed it. I have not seen any such requirement in my USAA loan docs/