I know exactly how this went down. The belay is protected by the overhang. When I heard the shee going down above I could not help sticking my arm out above the overhang for balance and looking up to figure out what was going on. I'm sure she did the same thing.
I was on the route 9 days ago, and the flake was still there, still looking precarious.
I peered underneath it for a while, and it looks like its kinda cammed into place by the bigger flake above it and a small (maybe 3" by 4") chock behind it. It looks like that chock is in there purely based on friction, and if it slips a little bit, the camming action that holds the death flake on will stop and it will go.
My partner led the king swing and then back-cleaned his way up to that belay. We were leading in blocks, so he took the next lead as well and I sat there under that thing for quite a while as I was belaying him.
Here's my advice on it
#1 - make super super sure the leader does not kick it or touch it as he goes by.
#2 - as the leader is leading that next pitch its up to the belayer to make sure ropes stay away from it. If a rope (lead or haul or tag) were to get behind it and then the leader were to suddenly pull up to clip, or take a whipper and have the rope come tight, that could be enough to dislodge it onto the belayer.
#3 - I rode our pig at this belay to get as far over to the left as possible so that if it did go, I wouldn't be directly under it and maybe it would go right by without hitting me or our bag. Not as comfy to sit on the bag like that leaning over to the left, but I think it was worth it.
#4 - follower has to be super super careful to not hit/kick the flake, or let a loop of rope lasso it while jugging.
#5 - don't haul to the hanging gear belay that supertopo recommends. Go up to the 3-bolt optional belay. If you have a 60m lead line the leader can pretty easily make it over there, and you only need a 50m haul line. Then when you haul, the bag will be heading out to the left. If you were to haul up to the supertopo hanging gear belay, you would be hauling right over the death flake... Plus who wants to do a hanging gear belay when there's a nice blocky ledge with 3 bolts on it?
We actually brought along a piece of webbing to try and sling it and see if we could maybe provide some additional support to it, but we ended up realizing that was harder than we thought, and our webbing wasn't long enough. Honestly we didn't spend very long trying, so maybe a party who was willing to spend more than 2 minutes could possibly sling it and provide a measure of additional safety.
There is another option, just run the pitches together. I did that, not because I was brave but because my partner told me to. He had got the beta from someone that it was no big deal. After the swing you climb about 15 ft with no pro then you can put in gear for the rest of the crack.