Researching work benches will often show many examples of the sliding deadman. I opted not to put one on my bench because I valued drawer space more.
On the rare occasion when I need to support a piece from the bottom I'll just use a clamp in expansion mode.
Well I'm glad someone uses clamps in expansion mode. I never have.
Is your benchtop a benchtop on top of a benchtop?
Also, excellent job on the seat carve. How did you do it? I've gone at mine with a grinder and a palm sander and... it's difficult to get good results.