On the Dreadnaught after the 1.28 update, I am able to go under the floor of the mausoleum. I just need to go to the far left corner drop down to the little wall rift that you can stand on, then face the floor and there is a small hole. You crouch then move in, then you can walk left along the overhang. Once there I just had to kinda finagle my way into the space between floor and nothing and now I can walk freely under everything in the mausoleum. Its kinda fun, not really hazardous it seems, just hope that Destiny fixes it or something.