I can find many vague references to the fact that you must be a member of a climbing association to use the sport climbing crags in Belgium (e.g. Freyr). But I can't find anything on the BMC website about what you actually need.
Is usual membership enough? Must you have a reciprocal rights card, or is that only for alpine huts?
For comparison, the dutch climbing club (NKBV) have a clear policy: members must buy an additional pass to climb in Belgium.