I was part of a group of nine people on seven bikes crossing in to Mexico a few years ago. The group were American and Canadian, and yours truly Australian. The bike I was riding belonged to a Canadian who was with us and had papers for 'my' bike.
No go..the Mexican authorities were sympathetic to us but could not find anything in their manual to accomodate me. We waited several hours for the head honcho. He shook his head, which I took to mean 'no go' in any language you can think of.
But then...as it happened, our friend and unofficial leader of our group (who spoke fluent Spanish) mentioned that he was born in Argentina....voilà, the magic key. We got out of there as fast as we could.
No money changed hands and there was no aggravation of any kind.
What I learnt was; never give up (but I knew that already), don't get upset, and where there is a will there is a way.
And so began one of the most enjoyable and happy trips I've ever been on.