I believe this has come up through a Support issue that was discussed at the end of last week, and the answer is that through XOG, no, you cannot read/write the proxy user settings of the User object.
You could create an NSQL Query to read (via XOG) the current settings, but NSQL Queries are not available for write operations and the understanding is that you wanted to change this information too, so it's not applicable as a solution.