While doing some additional research for this question, I found out the answer already, but it's perhaps worth sharing anyway. It turns out my assumption
Dutch number plates never have vowels
was slightly wrong; even though Wikipedia supports that statement:
Nowadays the letters used do not include vowels, so as to avoid profane or obscene language.
it apparently doesn't hold for semi-trailers whose number plates start with the letter O, and the opening is indeed to indicate that it's an O, according to the responsible government organization (link in Dutch, my translation follows):
The letter O in the registration identifier
If the registration identifier contains the letter O, then the letter O on the number plate should be printed with an opening on the upper left side.
As a letter and a number can't appear together in one component (there is always a dash between them), there's no real risk of mistaking an O for a zero.