For the Brussels show it will be hard to get a ticket, concert was sold out in about 1 hour... I took a half day vacation just to be sure I had my ticket.
It's a rather small venue (about 2000 people) so I guess most of them will be eager to see them play.
If you're lucky, maybe someone could sell his ticket, check the forum of the sonic youth concert (just go to the concert page and check the comments) at
www.abconcerts.be and maybe you get lucky.