I guess a better question is how does this component know a validation has failed? How can I set that in the controller so it shows "errorMarker" instead?
It works fully the same as standard h:messages except additional ajax requests handling. So seems you need to explore general JSF docs about this ;)
This was more of an issue of what I was using as the fieldname. It turns out you need to use the fully qualified name (ie., formName:fieldName) and it is case-sensitive.
Maybe a help for others...