I found a method that seemed to work almost everytime so I decided to share.
Although I have already freed all the fortress' from Templar hold I could still get a convoy to appear.
Go to the Fort Duquesne area in Black Creek, follow the main road up to almost the entrance of the fort then stand aside. Wait for the Peddler mark on your big map to disappear and start walking along the road leading towards Concord, somewhere along the road you should get a notification to attack the convoy some 50 meters before the convoy and most likely during the tight turns at the border of Black Creek and Diamond Basin.
I've attached a map to show where the convoy should move.
The trick is to sneak up on them and choke as many of the Redcoats as possible, it doesn't count as killing.
Some have said that executing a combo with a kick included will actually kill the soldier and therefore corrupt the achievement, so try not to finalize combos.
Also use loads of smoke bombs after being noticed, you'll then be able to sneak up on the soldiers and choke them, keep in mind they might not work on the captain.
Calex dEUS noticed that if you haven't already been invited to the Boston Brawlers you might get a guy running up to you in the middle of the attack on the convoy and cause the achievement to glitch, so try to do some unarmed kills before trying, just in case :)
RE2Leon and Impdaddyzer0 mentioned that using an enemy as a human shield against the captain whom will almost certainly pull out his pistol and fire at you will result in a failure, since if anyone dies you're screwed.
Koolkong94 notified me of the fact that poison darts are non-lethal. You'll just have to wait for the poison to take full effect.
The BlackxRanger tells me in the comments section that you can actually counter with melee, as long as the guards don't hit trees or cliffs afterwards. Can't confirm this myself since I haven't got the game anymore, but it seems legitimate. As long as they roll on the ground after you've downed them you should be fine!
Some also say that throwing an enemy against a tree, wagon or a rock will count as killing him so try not to do that either.
The map is courtesy of IGN.
The border isn't accurate either.