I think I've seen a question about this topic with an screenshot of a entire hierarchy, I'm trying to do something similar myself and I'd like to review the experience other developers had.
Please take a look at this example described in PI Web API Programmer's Reference.
Getting Started with the PI Web API - A Simple Application
It is quite possible to get a full element hierarchy through the PI WEB API. you might initially have to specify either the unique web ID of the initial element or the path of the initial by which you can get the web id of the element. And form that you can easily derive the entire hierarchy through some basic querying of the PI WebAPI.
Thank you, I'll try this and see how it goes.
Retrieving data ...