Interesting concept!
I was not familiar with this exam system before you posted, so I can’t definitely answer your question. It does look like there are no rules re variety. Did you see the contact us section? There is an individual email and phone number for each region. I would hope they could give you a definite answer.
If you decide to do this, I hope you will post your impression when you finish. In particular, I would be interested in the level of comment detail provided by the examiner, and whether it was one or multiple. This could be useful feedback but it’s not cheap!