Kittens are very adaptable. Sona learned immediately to be careful with jumping up to an open toilet, Juku took two dips before learning. By now they are old pros at jumping up to the seat without falling in.
Phase II and III: Moving and raising the litter box
Since the litter box was in the bathroom already, I didn't have to move it there, I just relocated it next to the toilet and raised it at the same time on Saturday after observing both of them successfully jumping up to the toilet seat independently.
Elevating the box was supposed to be done in stages, but I didn't have anything suitable to put under it. It is lidded, and the lid had indentations for the box feet, so I just lifted the box onto the lid. Voilà! A raised litter box.
I lifted them up to the box to show where it was, and it was smooth sailing afterwards. It's not as if they weren't there observing the whole process, or that they wouldn't be able to smell the litter in the new location. I'm confident they would have figured it out by themselves, but I believe it is a good idea to show an animal you are training what you expect them to do.
So far, so good. Things will get interesting now. Leaving a toilet seat up or relocating a litter box is nothing unusual, getting them to associate the toilet as a place to do their business is the crucial bit. Phase IV will start when I find a suitable bowl.