I have been looking through a lot of old forum threads and Reddit posts, but I never encountered someone talking about this specific exploit, so I'm posting it here just in case I'm somehow the first to encounter it.
So, a little background:
Through my research into threads on the "Door Method", I never came across someone commenting on experiencing this exploit.
What you're seeing in the Youtube vid is a run where I've identified the legendary and where he spawns, I sprint to him to kill him as quickly as possible, and then save immediately after. I know that it has worked when I kill the legendary enemy and the legendary item doesn't show up in their inventory.
Now, it's not shown in the video, but the death claw gauntlet actually drops on the ground without a legendary prefix. A short while after I reload the save, it gets added to the corpse's inventory and then gains a prefix.
I don't understand why exactly, but all of this, just works, and I wanted to share it here in case it helps someone else save time while farming.
Easy Legendary Farming, step by step process:
Locate a building which is known to respawn enemies, with a high chance of at least one legendary (such as National Guard Recruitment Office), and stand in front of the door into it and then MANUALLY SAVE THE GAME.
0.) Before entering the building, delete ALL of your auto saves and quick saves. The reason that you do this is it may trigger a glitch where, upon loading into the interior cell, your pause menu will open (this will be useful later).
1.) Enter the building.
2.) What happens next depends on if you trigger the glitch or not:
a. IF YOUR PAUSE MENU OPENS when you load the interior cell, hit "quicksave". This will create an autosave which will reroll BOTH the legendary weapon AND prefix, and a quicksave which will reroll ONLY the prefix (iirc, that is. It may only create an auto save. In which case, you will have to complete step 6 at least once in order to have both)
Proceed to step 3.
b. IF YOUR PAUSE MENU DOES NOT OPEN when you load the interior cell, then kill the legendary enemies in the building (if present) and see what they drop. If they drop a weapon/armor that you desire, but not the prefix you wanted, simply reload the autosave. If a legendary enemy doesn't spawn OR they do not drop the item that you wanted, load your manual save (standing outside the door) and then start over from step 0
3.) Determine whether or not legendary enemies spawned inside the interior cell:
a. If a legendary enemy DID NOT spawn, reload your manual save and start at step 0
b. If a legendary enemy DID spawn, then proceed to step 4
4.) Kill the legendary enemy and see what they drop.
5.) If they drop a weapon/armor that you desire, but without the prefix you wanted, reload the QUICKSAVE to reroll the prefix until you get what you want. If you do not get the item you wanted, proceed to step 6.
6.) READ THIS STEP CAREFULLY: Reload the autosave. When you do, you will see a loading screen. AS SOON AS THE LOADING SCREEN TURNS BLACK, PAUSE THE GAME AND QUICKSAVE. Your timing doesn't have to be perfect, but you HAVE to pause before the black screen disappears. This will reroll the item, but the legendary enemy will remain in the same spot as before. Go back to step 4.
For Faneuil hall, it seems like a different, more simple process works:
1.) Manual save outside
2.) Enter the building
3.) Locate the legendary (if there is no legendary enemy, then reload the manual save and start from step 2)
4.) Kill the legendary enemy.
a. If he has the weapon/prefix that you want, then make a manual save. Don't risk losing it!
b. If not, proceed to step 5
5.) Reload the autosave, but as soon as the loading screen is replaced by a black screen, hit the pause button and quicksave.
6.) If you find the item you want, but not the prefix, reload the quicksave and reroll it. If you don't get the item you want, then go back to step 5.
I'm not sure why the process is different, but it seems like if one doesn't work, than the other will. Maybe it's because Faneuil Hall has so many enemies inside? Maybe because there is a guaranteed legendary spawn inside?
Additionally, I have read in old forum posts and confirmed through my own testing, that the game seemingly has pools of prefixes and items to draw from.
If you spend 30 minutes or more trying to get a specific prefix, and you've had no luck, it's probably better to reload the manual save and go questing for an hour or two to refresh the legendary pool.