The Hidden Blade Assassin's Creed in Depth

Note: Aurel found this first and then told me about it, so I didn't really do anything.
You can do this by starting the William assassination at the bureau and then infiltrating the fortress without the scene starting (see this article). At certain points in the fortress, all the soldiers will disappear. You can then setup an 'invisible kill' with air-to-assassinate: you lock onto your target and make sure you jump through one of these spots. Near the back of the group is a great spot.
Altaïr then performs an air-assassination ( ) and the memory corridor scene starts. Afterwards, you can just stroll out of the area and get back to the bureau, after which you will have completed the mission.
I opted to get out of the city instead, to see if the back-to-the-future exploit worked. It didn't, but William's army has become substantially smaller, which makes setting up an anonymous kill very easy with a couple of knives.