Funny thing, just today I came across the same issue. 
So far I came up with an a little bit refined version of the OP's suggestions. Until now I have tested this with Python-2.7.9 and tk-8.6.0 on debian Jessie, I think the same code should work for Python-3 , too.
The busy_current() function here always reports only the root window (the same when called from within wish, apparently a bug in this early version of Tk-8.6) so I was not able to figure out how to use and test the "pattern" option.

    def busy(self, **kw):
        '''Shortcut for the busy_hold() command.'''
        self.tk.call(('tk', 'busy', self._w) + self._options(kw))

    def busy_cget(self, option):
        '''Queries the busy command configuration options for
        this window.
        The window must have been previously made busy by
        the busy_hold() command. Returns the present value
        of the specified option. Option may have
        any of the values accepted by busy_hold().'''
        return(self.tk.call('tk', 'busy', 'cget', self._w, '-'+option))

    def busy_configure(self, cnf=None, **kw):
        '''Queries or modifies the tk busy command configuration
        options. The window must have been previously made busy by
        busy_hold(). Option may have any of the values accepted by
        busy_hold().
        Please note that the option database is referenced by the
        window. For example, if a Frame widget is to be made busy,
        the busy cursor can be specified for it by :
            w.option_add("*Frame.BusyCursor", "gumby")'''
        if kw:
            cnf = _cnfmerge((cnf, kw))
        elif cnf:
            cnf = _cnfmerge(cnf)
        if cnf is None:
            return(self._getconfigure(
                        'tk', 'busy', 'configure', self._w))
        if type(cnf) is StringType:
            return(self._getconfigure1(
                        'tk', 'busy', 'configure', self._w, '-'+cnf))
        self.tk.call((
            'tk', 'busy', 'configure', self._w) + self._options(cnf))
    busy_config = busy_configure

    def busy_current(self, pattern=None):
        '''Returns the widget objects of all widgets that are
        currently busy.
        If a pattern is given, only the path names of busy widgets
        matching PATTERN are returned. '''
        return([self._nametowidget(x) for x in
                self.tk.splitlist(self.tk.call(
                   'tk', 'busy', 'current', self._w, pattern))])

    def busy_forget(self):
        '''Releases resources allocated by the busy() command for
        this window, including the transparent window. User events will
        again be received by the window. Resources are also released
        when the window is destroyed. The window must have been
        specified in the busy_hold() operation, otherwise an
        exception is raised.'''
        self.tk.call('tk', 'busy', 'forget', self._w)

    def busy_hold(self, **kw):
        '''Makes this window (and its descendants in the Tk window
        hierarchy) appear busy. A transparent window is put in front
        of the specified window. This transparent window is mapped
        the next time idle tasks are processed, and the specified
        window and its descendants will be blocked from user
        interactions. Normally update() should be called immediately
        afterward to insure that the hold operation is in effect before
        the application starts its processing. The following
        configuration options are valid:
            -cursor cursorName
                Specifies the cursor to be displayed when the widget
                is made busy. CursorName can be in any form accepted
                by Tk_GetCursor. The default cursor is "wait" on
                Windows and "watch" on other platforms.'''
        self.tk.call((
                'tk', 'busy', 'hold', self._w) + self._options(kw))

    def busy_status(self):
        '''Returns the busy status of this window.
        If the window presently can not receive user interactions,
        True is returned, otherwise False.'''
        return((self.tk.getboolean(self.tk.call(
                'tk', 'busy', 'status', self._w)) and True) or False)