It would be very nice to have a function in pickle to
load an object without creating a new instance, but by
using an existing one, meaning that __init__, etc. of
an existing instance is called.

Something like:
updateload(object, file)

(I have no patch in mind.)

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I realize it might not be clear.  What is needed is simply a
function that does the same as pickle.load but that uses an
existing instance instead of creating another one.  All the
rest after that point is the same.  It means this function
would raise an exception if the instance is not of the good
type, etc.  (Forget the part about __init__).

Any real use cases beyond being very nice?

This functionality already exists.  

"""
If a string is returned [from __reduce__() method], the string should be interpreted as the name of a global variable. It should be the object’s local name relative to its module; the pickle module searches the module namespace to determine the object’s module. This behaviour is typically useful for singletons.
""" -- http://docs.python.org/dev/py3k/library/pickle.html

On the second reading, OP's request has nothing to do with pickle.load().  In fact, there is no coherent request here.