Hello I'm new here. Thanks for making such a great game, I really love it.
The main problem with exporting .map from blender is extremely large number of brushes. I`ve tried to make and edit some objects, and result wasn`t good. May be I`ve missed something.
I just tested this and it seems to work fine. You just have to keep in mind that you must use brushes in Blender. Try to model with separate, closed boxes, like you do in the Radiant, then it'll work.
Now I wonder if texturing in Blender works, too. Gonna check this out later.
Edit: OK it's more difficult than I thought. Exporting cubes works fine, but you can't export prisms and cones without creating lots of unnecessary brushes. I also tried to export NURBS surfaces which didn't work at all. Are the curves in the Radiant NURBS?