Introducing the Horse to Armor
Training for the joust and introducing the horse to armor are two different things, and should be done at two different times before they are combined.
When you are not jousting
Add armor to your daily routine before you add it to your jousting practice. Don't "sack them out" by throwing everything at them at once, but rather associate armor with something positive like treats.
Put someone into armor, and have them deliver carrots. Eventually the horse will disregard the armor and focus on the carrots. Five minutes of horses chasing armored guys around to get carrots is better than two weeks spent lunging a horse with armor tied to the saddle, and more effective.