Have you tried some sort of lock-off right beyond dead-hang? It is generally quite difficult to lock that off on an negative/eccentric pullup once your elbow angle gets bigger than 90 degrees.
I dunno about the pullup shrugs. I use them in my WU before dead hang holds. No idear. My shoulders like lots of warmup.
So perhaps you can rig up some sort of assist that will allow you to break from the deadhang and hold. Or work that lockoff from the descent with an assist. Feet on a block or band assist.
Maybe weighted body rows?