It is stuck between the tank and the bowl, yes. I have not tried to use a plunger inside the tank, but I'm dubious that a plunger will successfully push the thing out through those little holes in the bowl's rim.
Please tell me this is sarcasm. At any rate, a plunger is unlikely to be effective in sucking the rubber back out since you're only pulling with air at that point. The rubber doesn't sound to be completely air-locked into the toilet, so it's unlikely to come out. Removing the tank is your best bet. If not, full-on toilets aren't terribly expensive.
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