mp5stab
Hair and Nails
I never heard of such method, so it is pure fantasy, but I think that the big nail - it must be really long and thick - hammered under the crotch would be very efficient as a sedile.
I've drawn an idea like that before, using one of the spikes as a sedile:
I think whether or not it was a historic reality matters less than whether or not it seems plausible as a method of prolonging an execution.