For what it's worth, I'm trying to do something similar. I've got the export script for Blender to ASE working, but loading objects in GtkRadiant was giving me a "NO SHADER FOUND" message. I solved this by editing the ASE file by hand and changing the MATERIAL_NAME for my material to "textures/arachnid2/dirt_1" (ie just some texture/material that's in the game already--if you're adding textures of your own, this will be different--just be aware that the Blender->ASE exporter will expect material names to be path names of textures/shaders you can use in GtkRadiant).
You can then insert these exported objects by right-clicking, saying "miisc->misc_model" and importing an ASE that way. I think this will just create a displayable object (you'll have to be more clever to have it create walls that resist the player, etc).
I'm not home free, though. My hideous object shows up in the editor, but there's no trace of it in the created BSP (or when I load the level in Tremulous)--it's like it isn't even there. So I'm obviously missing something about that.. At least it's a start.