Seems some of the answer might depend what bike you've got ... what it's worth and what you've got into it and how much you expect to get for it.
You're correct, it can't be registered in USA (probably not Canada either).
I assume you've got all the original Chilean paperwork? If yes, do some really well done Photo Shop work filling in the buyers details, create new title. This, in theory, should work.
In most of Latin America they'll not have ever seen a Chilean title, so a copy should be fine for passing borders if it looks reasonably official. (good paper and such) Mostly they just want the Cash FEE for your TVIP and a valid passport.
Once in Chile the buyer could (in theory) take the original title with a notarized statement and legally transfer ownership to his/her name. Maybe.
If the bike is worth near nothing then maybe not worth the trouble. You could give it away but recipient would still have issues to show it's his and legal and would not be able to get a TVIP without title in his/her name and could never register it in USA or Canada.
USA is lax on such things and the buyer could likely ride around forever using your paperwork and Chilean plate, just saying it's borrowed. As long as it's not reported stolen, this is fine ... and legal. I assume you entered USA via land border from Mexico. So no USA issued TVIP, correct? And no paperwork at all concerning your bike? Correct?
He/she would need minimal liability insurance. Local USA cops don't care much about foreign plates with foreign riders riding them or registration and such. As long as rider and bike are not wanted and law abiding, usually all OK.
If buyer is American and gets pulled in his state of residence (ie: California) then cop will write a ticket saying the bike must be registered in California within 30 days. So now the game is up. But if a foreigner or out of state tourist, no worries, Chilean plate is legal, but insurance is a must have.