HierarchicalTaxon.svc 401 for Authenticated User

I'm trying to use this service within a HierarchicalTaxonField on the frontend of the website.  If I bind on the server, the data populates, but a selection can't be made due to a javascript error on the get_selectedItems() method--apparently the object invoking the method is always null.

If I bind on the client, the data won't populate for an Authenticated User, unless the account is also granted Backend User permissions.  Once Backend User permissions are granted, though, the control works flawlessly.

I've tried EVERY permission on the custom hierarchical taxonomy to allow a simple, Authenticated User (not backend user) to get data bound on the client to no avail.  Please tell me that I don't have to grant Backend User permission to thousands of accounts just to bind data on the client. I need this to work without Backend User permissions.

I beat this, but I had to rebuild everything to do it.  Since the HerarchicalTaxon service checks for Backend User permission, I had to create my own data source for the control.  Since the HierarchicalTaxonField control doesn't set the ServiceUrl property of its RadTreeBinder child control to its own WebServiceUrl property value, and it uses a private const string (can't override) for the "predecessorWebServiceUrl" property, I had to create a custom control and template. Without the "service url" properties being overridable, the standard control reverts to standard HierarchicalTaxon service and the permission problems become an issue again.

This is a lot of trouble to let an Authenticated User select a custom hierarchical taxon/taxa via a slick and localizable interface. On the other hand, it beats the heck out of starting from scratch.