We've now done two practice tantrums and I can see why it was recommended not to do them too frequently. It's hard to work your child up to a point of actual upset (and otherwise they're not learning to control when they have strong emotions). It goes against every instinct as a parent.
Nathan always gets lots of cuddles when he's done. And I make sure there's a wide amount of time before he'll have to do anything.
We haven't had any further incidents of hitting reported from school. We had one incident here, which was promptly given a time-out and loss of screen time.
I've told Nathan that I don't like doing the practices, that I don't like seeing him upset. But I am proud that he's learning to control his temper and I know how hard that is. Hopefully the lesson will be learned quickly and I won't have to do it for long.