Maybe looking at it will help you decide how it will come out.
Well, there's no reason that should have helped me - I had looked at that fiche many times. But nonetheless, looking at it again, a light went on.
I could see the end of the sensor through the top tank hole (with the chrome piece removed), and I managed to slip the o-ring over it using some needle nose pliers, then worked it downward. It took some finger contortions and I wasn't sure I would be able to do it. It could have been a disaster given how clumsy I tend to be - dropping the o-ring into the tank was a real possibility, but it worked out.
Thanks!