org.zkoss.lang.reflect.Fields.get/set() does not directly support maps.
org.zkoss.zkplus.databind.Databinding.fetchValue() has some workaround like
if (bean instanceof Map)
bean = ((Map)bean).get(nodeid);
bean = Fields.get(bean, nodeid);
But other classes do not have this workaround, e.g. FieldComparator
Therefore <listheader sort="auto(keyMap.2.value)" /> does not work.
I would suggest to support map access directly within Fields.get/set() methods!
This would provide a consistent behavior for databinding + sorting + etc.
Please see patched class org.zkoss.lang.reflect.Fields to get more details.