I managed to consume the XML-RPC service from a Web App. I use https://github.com/timheap/jquery-xmlrpc

To make it work I had to modify the file <Odoo> /openerp/service/wsgi_server.py in the following procedures

def xmlrpc_return(start_response, service, method, params, string_faultcode=False):
    """
    Helper to call a service's method with some params, using a wsgi-supplied
    ``start_response`` callback.
    This is the place to look at to see the mapping between core exceptions
    and XML-RPC fault codes.
    """
    # Map OpenERP core exceptions to XML-RPC fault codes. Specific exceptions
    # defined in ``openerp.exceptions`` are mapped to specific fault codes;
    # all the other exceptions are mapped to the generic
    # RPC_FAULT_CODE_APPLICATION_ERROR value.
    # This also mimics SimpleXMLRPCDispatcher._marshaled_dispatch() for
    # exception handling.
    try:
        result = openerp.http.dispatch_rpc(service, method, params)
        response = xmlrpclib.dumps((result,), methodresponse=1, allow_none=False, encoding=None)
    except Exception, e:
        if string_faultcode:
            response = xmlrpc_handle_exception_string(e)
        else:
            response = xmlrpc_handle_exception_int(e)
    start_response("200 OK", [
                              ('Content-Type','text/xml'),('Content-Length', str(len(response))),
                              ('Access-Control-Allow-Origin','*'),
                              ('Access-Control-Allow-Methods','POST, GET, OPTIONS'),
                              ('Access-Control-Max-Age',1000),
                              ('Access-Control-Allow-Headers','origin, x-csrftoken, content-type, accept'),
                    ]
                )
    return [response]
def wsgi_xmlrpc(environ, start_response):
    """ Two routes are available for XML-RPC
    /xmlrpc/<service> route returns faultCode as strings. This is a historic
    violation of the protocol kept for compatibility.
    /xmlrpc/2/<service> is a new route that returns faultCode as int and is
    therefore fully compliant.
    """
    if environ['REQUEST_METHOD'] == "OPTIONS":
        response = werkzeug.wrappers.Response('OPTIONS METHOD DETECTED') 
        response.headers['Access-Control-Allow-Origin'] = '*'
        response.headers['Access-Control-Allow-Methods'] = 'POST, GET, OPTIONS'
        response.headers['Access-Control-Max-Age'] = 1000
        # note that '*' is not valid for Access-Control-Allow-Headers
        response.headers['Access-Control-Allow-Headers'] = 'origin, x-csrftoken, content-type, accept'
        
        return response(environ, start_response)
    
    if environ['REQUEST_METHOD'] == 'POST' and environ['PATH_INFO'].startswith('/xmlrpc/'):
        length = int(environ['CONTENT_LENGTH'])
        data = environ['wsgi.input'].read(length)
        # Distinguish betweed the 2 faultCode modes
        string_faultcode = True
        if environ['PATH_INFO'].startswith('/xmlrpc/2/'):
            service = environ['PATH_INFO'][len('/xmlrpc/2/'):]
            string_faultcode = False
        else:
            service = environ['PATH_INFO'][len('/xmlrpc/'):]
        params, method = xmlrpclib.loads(data)
        return xmlrpc_return(start_response, service, method, params, string_faultcode)

Would you know tell me how to take these changes into an installable module? Would I have to go into the core?