A somewhat effective way to use it would be to stock up on scales for a while, then sell them to others.
If war or "friction" was to come about, the emperor could set up a "trojan horse" styled attack on his neighbor. The dragon could be given a drug to put him to sleep, then armor him up and quickly deliver him within a large structure with the gold scales also in, as a peace offering to his enemy. Then, when the dragon wakes up, he gets mad and burns down the enemy kingdom. The armor will protect him from hostile attacks.
Getting the dragon BACK would be a challenge, but shooting the drug into it from a distance and hauling him back could suffice (or however our emperor caught him in the first place.
List of natural sedatives:
Lavender seems to be a relatively simple choice, but any could have been used in ancient times as a sedative.