I've been thinking on this bucking bar thing. First, the length I see repeated is 28mm. At the very least, stainless steel. Perhaps the shoulder off a longer bolt. Grade 5, 5/16". Or Grade 8 - I use Grade 8 whenever possible. You could also use the base end of a hardened 5/16" drill bit.
The clutch on these HT motors is pretty simple and dependable. The only weak spot iOS the bucking bar / cam interface. You have to have a clutch cam in reserve ready to go at all times.
Ideally, I'd envision a kind of hydraulic activator that would simply push the bucking bar in to disengage the clutch. It would have to be small and dependable.
Just me thinking out loud.