Because I'm checking out icerinks today and I feel some nostalgy for the open air BLOSO icerink in Herentals because I went there in my schooltime (12-15 years old), I came across this:
http://www.bloso.be/public/centra/Herentals.aspOn this page they mention an indoor "gaanderij" hall and inlineskating is mentioned explicitly as a possibility!!! Checking the prices for this "gaanderij" it is 7.20 Euro/hour. Very reasonable, I think.
So, Stijn and Evy, I'm beating you in your own area! Could you maybe have a look at this place? Take some pictures? Find out how large it is (place for 500 people to watch!)? When it would be free?