If you arrive @ 5am, you just miss a bus to antwerp, which runs hourly.
You can catch a ride in most cases with some other yidden.
Catching the 6am bus, is a option, but you will have to be fast, to catch the 8am bus to return, but that's kinda risky.
Taxis (not the regular ones,) take 50 euro per direction.
To sum it up, if you can get a ride to antwerp, ask to be dropped of by gerrer shteeble, I believe they have a 6:30am minyan (drive from airport is about 45 mins with a car at this time).
An option would be for someone to drop you at the central station (7 min drive), and take a train to BRU, which is estimate that you will arrive at 8:30am.