First post here, also sorry if this is a repost.
I went to do Siegecrafter on LFR on my rogue, jumped on the belt and tagged one of the weapons then accidentally ran into the dark area on the belt and died. I was able to loot Siegecrafter and coin as per usual, I ended up getting DC at paragons so I got kicked and decided to queue again, but this time I noticed that it didn't say I looted Siegecrafter.
I queued again and repeated what I did before, and again I was able to loot Siegecrafter, I don't know if this would work on any other bosses.
So basically, jump on the belt, tag a weapon on the belt, die on the belt via flames or getting to the end of the belt and you shouldn't get loot locked.