Ok, I investigated this a little further. First I noticed another bug with the code from my first post, the "self._w" must be omitted from the call to busy_current(), so the func should look like:

    def busy_current(self, pattern=None):
        return([self._nametowidget(x) for x in
                self.tk.splitlist(self.tk.call(
                   'tk', 'busy', 'current', pattern))])

I did then some more testing, the code now seems to work flawlessly both with Python-3.5.2 on windows 10 and with Python 2.7 on debian Jessie. So I finally prepared a patch (against the __init__.py file from the 3.5.2 windows installer). Following Miguel's suggestion I also added aliases prefixed with tk_ to the various busy_... methods.
Since I am not familiar with Python's "official" test mechanisms, for now I wrote a simple script that does a number of calls to the tk_busy_... functions to test if everything works as expected:

try:
    import Tkinter
except:
    import tkinter as Tkinter
#Tkinter.wantobjects = False

root = Tkinter.Tk()
f = Tkinter.Frame(root, name='f')
f.pack(fill='both', expand=1)
b=Tkinter.Button(f, name='b', text='hi', command=root.bell)
b.pack(padx=100, pady=100)

top = Tkinter.Toplevel(root, name='top')

def test1():
    root.tk_busy()
def test2():
    root.tk_busy_forget()
def test3():
    root.tk_busy_hold(cursor='gumby')
def test4():
    root.tk_busy_forget()
def test5():
    root.tk_busy_hold()
    top.tk_busy(cursor='gumby')
    print(root.tk_busy_current())
    print(root.tk_busy_current(pattern='*t*'))
def test6():
    print(root.tk_busy_current())
def test7():
    root.tk_busy_configure(cursor='gumby')
def test8():
    print(root.tk_busy_configure())
    print(root.tk_busy_configure('cursor'))
    print(root.tk_busy_cget('cursor'))
def test9():
    print(root.tk_busy_status())
def test10():
    root.tk_busy_forget()
    print(root.tk_busy_status())
    print(root.tk_busy_current())

delay = 0
for test in (test1, test2, test3, test4, test5, test6, test7,
                test8, test9, test10):
    delay += 1000
    root.after(delay, test)

root.mainloop()