This is probably not a good idea.
First due to the nature of the activities that take place in a human bathroom, there are places that the average human does not come in contact with on a regular basis that can collect liquids and germs. These types of places, like the seam where the toilet meets the floor, are the type of places that rabbits like to explore. Rabbit systems are delicate and foreign germs can devastate a rabbits system, and potentially causing death. Just out of an abundance of precaution I keep my rabbits out of the bathroom.
Another concern is that rabbits are not as agile as cats. This means that a rabbit that loses its balance while on the seating area is much more likely to find themselves in the basin with the water and any unflushed waste. Domestic rabbits are not used to dealing with being wet if you are not there to help clean and dry them this can lead to other issues with their fur, and potentially cause the rabbit to go into shock.
Rabbits like to eat hay while in their litter box. Successful litter box training nearly always includes a hay source positioned so that hay can be consumed while producing droppings. Stray or imperfect hay migrates to the floor of the litter box, where it goes to compost with the rest of the litter. Assuming you train the rabbit to use toilet (conceivable), if you fail to include hay (a dietary requirement) at the toilet the rabbit will prefer to use the facilities where the hay is. If you do include hay at the toilet, your plumbing system is unlikely to appreciate the extra fiber.
In addition there is really no practical reason for this. Most bunnies are clean and will leave their potty in the same place most of the time. If you provide them with a litter box to do this they will leave most of their mess there. Any problems you have with droppings left in other places are not going to be alleviated by training the bun to use the toilet. They are most likely either territory marking or the fact that rabbits do not have the control like humans do. Neither of these is going to change if you can manage to train your rabbit to use the toilet.
Lastly, rabbit dropping are make a super fertilizer when composed that is hard to top.
