Hi Markus,
Thanks for you message, which we have read with great interest.
In principle all pilots are welcome to fly in Belgium, whether individual or in a group. Still, we do have our ATC procedures and we have the general IVAO rules to respect. Like every where else 
There comes the problem, since we do understand that you have in mind to try to land with a maximum of 12 pilots preferably all at the same time .... That is impossible, whether it is IVAO or real life.
When you will enter the BE FIR, we take it you are duly separated as it should be, or anyway ATC has to take care about that and will arrange for a proper sequencing, so that a safe landing sequence is assured.
With such sequencing ATC might influence your exercise and without doubt does effect the landing time for all of you. Landing will be performed as usual one after the other with sufficient separation. Be advised, ATC determines the sequence. However, he cannot be held responsible in case any one would disagree with his judgement. This will be solely based on the traffic situation, as usual.
Furthermore, seen the idea of your competition, we propose to use the airfield of Oostend (EBOS). It has similar facilities as Antwerp, but the approach procedures are less restrictive. In addition, it has a much longer runway and more turn-offs. More importantly, traffic density is less in EBOS and since it's more isolated situation, it will be more suitable for our convenience to accommodate your group.
Finally, as mentioned before, we do respect the common rules and procedures of IVAO in Belgium and we do expect that pilots flying in our FIR do the same. They shall be able to follow the rules and the instructions from ATC, as it should be. If not, there may be a supervisor on duty, who may take proper action.
So, please let us know asap, whether or not you can agree to the above and consequently, whether we may welcome you Wednesday night at EBOS. In any case we will do our utmost to provide adequate ATC.
Hope to hear from you soon.
Kind regards,
On behalf of BE Division,
Bob van der Flier
Kindest regards, Bob van der Flier, BE-ADIR & IVAO-ATA4