Did Mesoamericans know that chocolate was poison to dogs?

I've never seen any evidence of pre-modern awareness of the toxicity of chocolate. Considering pre-Columbian chocolate, I don't expect there to be any, either.

Chocolate, or more specifically, theobromine, the caffeine-family chemical in chocolate, is toxic, to humans and other mammals, as well as to dogs. The LD50 (the dose at which half of subjects are expected to die) of theobromine for humans is estimated to be about 1000mg/kg of body weight. For a 75kg human (approximately the average male adult weight), this would mean eating about 7.5kg of 65% dark chocolate, or 7.5kg of cacao beans (about 7000 beans), or 2.5kg of cocoa powder. To consume even close to this would take significant dedication, and for the pre-Columbian case, it would also be very expensive - about 175 days wages for a labourer, or enough to support a person for about a year and a half.

Theobromine is more toxic to dogs than to humans - the LD50 for dogs is about 300mg/kg. For a small Mexican dog, say 2kg in weight, this would require eating about 55 beans. A dog that ate 1.3 days wages in one sitting might find itself turned into hot dogs before the toxicity of theobromine made itself evident!

Another important factor is that sweetened chocolate is a modern invention. Pre-Columbian chocolate was unsweetened, and bitter. The smell of cacao beans isn't attractive to dogs - for example, there are very few cases of theobromine poisoning of dogs due to cocoa mulch (made from the shell of cacao bean pods, and smells of chocolate, and contains theobromine). The chocolate that poses the highest risk to dogs is sweet chocolate, which dogs will eat because it is sweet. (Cats are not so keen on sweet foods, and even though they are even more susceptible to theobromine, with an LD50 of about 200mg/kg, suffer far fewer cases of poisoning.)

Together, the lack of sweetened chocolate, and the expense of chocolate, would have made theobromine poisoning of dogs rare enough in pre-Columbian Mesoamerica that it would have been unlikely to be noticed.