Personally, I don't believe tonic immobility occurs in rabbits. Signs of tonic immobility are an increase in heart rate and blood pressure after TI, as well as a dilation of the pupils.
Rabbits prefer to have all four feet on the ground at all times. This is because, being prey animals, if a rabbit cannot run away they are likely to be attacked. A rabbit knows that it's only hope in the face of danger is running away. In the wild, a rabbit playing dead or being immobile is not observed. Rabbits don't play dead to escape a predator; they do almost the complete opposite; they run away.
It is stressful for most rabbits to have all their feet off the ground and being in on their back. It is a vulnerable position. Therefore, when most rabbits are held that way, they exhibit stressed signs like dilated pupils and an increased heart rate during and after being held on their back. This is generally the same way when most rabbits are picked up.
My rabbits grow up in a safe environment. When they are put on their back they are not terrified. They are relaxed. They grind their teeth in comfort, and their heartbeat slows. I feel their muscles relaxing. When a rabbit is tense, you can feel it.
Tonic immobility is, literally, an animal being paralyzed out of fear. When being turned on its back, rabbits are not paralyzed. Many owners can say that their rabbit will not stay on their back, that it's nearly impossible to try. I've had a couple rabbits like this as well (none of them born here), and it has worked for me to scratch their head and get them comfortable and they stay put for a couple minutes. Each of my rabbits will struggle if they are held on their back for long enough. But just for quick nail trimmings or a health check, they relax.
What are your thoughts on tonic immobility occurring in rabbits?