Many thanks for the thoughtful replies to my question. Successively, they pointed out that

1. I was, as so often, missing the obvious by not sub-classing
2. but that the creation of a new class implies, to me at least, a more stable setup than I wanted;
3. but then that there's quite an easy way to change a class-wide method definition, which was good enough for me
4. and that finally, there's a way of doing it the original way I had conceived anyway.

In the end, subclassing was easiest, and mostly fitted what I wanted to do. After all, it's a test script, which for me only needs to test elegant code, not be elegant code itself.

As an aside, the reason that I'd originally thought of approaching it by runtime method redefinition is probably a habit from Ruby (please don't curse me for mentioning it in this holy place). Ruby has a cute of way of doing this (and so, maybe will Perl6?):

class Dog
  def bark
    puts "woof"
  end
end

fido  = Dog.new()
rover = Dog.new()

class << rover
  def bark
    puts "meow"
  end
end

fido.bark() # woof!
rover.bark() # meow!
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Once again, thanks for your replies