shadowPseudoId() is being implemented in Element base class and returns a 'pseudoId' attribute which inspector also displays. But some derived from Element classes chose to override 'shadowPseudoId' to return their own pseudoId instead of setting an attribute. Inspector could probably query shadowPseudoId() to find out the pseudo id for those classes.
I'm not sure the inspector calls pseudoId itself, it just iterates over the attributes on an Element. The problem is that shadowPseudoId isn't exposed that way. InspectorDOMAgent::buildArrayForElementAttributes would have to special case pseudo, which is a bit weird. Instead, if there is a shadowPseudoId it could be exposed explicitly and displayed in the node tree differently.
AFAICT, this is shown now in inspector on trunk.