While the rabbit might look peaceful, the phenomenon is called tonic immobility, colloquially known as playing dead, and it's a fear-based response to predators that, for most rabbits, is involuntary and unpleasant.
Studies have shown that a rabbit's heart rate and blood pressure can actually drop while being tranced. While it can be a helpful tool to calm a feisty rabbit at the vet's office, it shouldn't be done recreationally - with or without a bath.
Of course, there are rare exceptions to the no-bath rule. Sometimes, despite an owner's best efforts, a mischievous rabbit will seek out trouble and end up, say, covered in chocolate syrup. Or an overweight or ill rabbit could be unable to clean himself.