As I was preparing for yet another business trip this week, it occurred to me that there will be no one to empty the small bowl. And then, that there would be no one to flush either if they were toilet trained. So, I gave up.
Considering that I travel often, that there are two cats, and even if a cat sitter flushed the toilet once a day, the frequency still wouldn't be enough. It would only work if the cat sitter lived in the flat and flushed the toilet as needed.
As far as progress went, they both put two front paws on the rim pretty regularly, it was random with Juku if he put two or three paws up. Sona only put two. It probably would have taken about one to two weeks more to completely train them, but...
They were so happy to get the usual litter box back. Happy and exited. I felt a bit bad for forcing them to use that tiny little bowl, but it was a good experiment. I just had not considered all the circumstances when I decided to begin.
To sum: it is perfectly possible to train cats to use a toilet. However, it is not practical if one travels often, or stays away for more than an ordinary work day. Even one night away is too long.