Can anyone recommend folk or traditional music venues in Bruges, Brussels, Antwerp, or Namur? We know about the festivals, but we're interested in pubs, concerts, etc.
I haven't been to any of those cities so I can't help. I do hope you get responses so I can file away the answers in case I get there. The local folk music is a highlight of any European trip.
Unlike in Germany, what you might consider "folk music" in Belgium often gets lumped together with stuff that might not seem "folksy", under the general category of "Schlager". If you're thinking of people playing acoustic and wind instruments, think again. It's more along the lines of hit songs from the 40s-60s being sung and performed in a somewhat kitschy style. I don't know of any specific venues, but look for flyers in places like frituurs and restaurants. Hasselt holds an annual "Schlagerfest", but I forget what time of year they usually hold it.
Another style of music that might be a little closer to what you have in mind is "Kleinkunst", which is sort of a fusion of the traditional European art song (in the tradition of Schubert, Mahler, Grieg or Wolf) with more modern instrumentation and arrangements. Unlike "Schlager", which is to art music what McDonalds is to haut cuisine, Kleinkunst usually aims for something a little more elevated than "I love you, honey."
And that being said, I have no idea where you would encounted Kleinkunst live, except on TV. My gut feeling would be that you would find it more in cities with a major university, like Gent or Leuven.