My understanding is that you would need to access from within the widget template to the Property XPath.
The idea is to allow your widget to have some info about the context of the target field.
In the current state there is no easy / simple way to fetch this information from the renderingcontext.
JSF components are bound to value, so only the value of the property is exposed not the property object itself.
However, this could make sense to add support for that in the framework. Since it requires to do some changes in the variable
resolver, this is not a quick tasks because it may have a lot of impacts.