replying to JA, Jim wrote:
CONFIRMED SOLUTION - I know that you are looking for and I do it all the time to
fill the bowl with water/vinegar to sanitize. Just buy a bag of medium-size
party balloons (should not cost more that a couple of dollars). Take one of the
balloons and fill with water until it is much larger than the toilet drain. Tie
off the top of the balloon. Now, carefully wedge the balloon into the toilet
drain hole, by manipulating the circumference of the balloon with your hands.
This may take a couple of tries, but basically you have to force some of the
water to the end of the balloon to get it to fit inside the drain hole. Tie a
string to the balloon and attach it to something just in case you forget and
flush the toilet - this also allows you to remove the balloon later, without
putting your hands in the water. That's all there is to it. The balloon will now
act as a plug in the drain hole and will keep water from automatically draining
out of the bowl. I do this all the time and it works like a charm. As I said, it
may take you 5 minutes of practice to get the balloon wedged tightly in the
drain - but I am sure you will get it.