The basic workflow for making maps from models is:
Make the map in Blender, cut it into sections (think visportals here), then export them as MD3.
Import the MD3s into Radiant then caulkhull, clip and hint.
Build the map as normal. If the model division and hinting were done properly then it shouldn't be significantly slower than a non model based map.
There are some Q3A maps out there that were made using this technique.
Its not a bad idea! GTK's tools are lame!
All tools are lame before you've learned to use them well.