Added a first, very restricted BSP loader for Blender 2.8x. Other versions are probably not supported because Blender keeps changing it's python API again and again. This probably requires changing add-ons every few months since there seems to be no compatibility between versions by design.
So this add-on can simply be installed via Edit -> Preferences, then Install... on the still zipped file attached. Afterwards activate the script in the same window. Once a map is opened, you will probably need to scale it down by a factor of 0.01 or something.
For 3D printing, you will need to solidify the outer walls as qbsp only keeps faces touching the worldspawn (not sure if this is the right terminology). So in contrast to .map files, a .bsp only contains polygon surfaces and not closed, convex brushes. For solidifying, I added a primitive shape and subtracted the map object from it. Obviously works best if the sky / walls touching the sky are all on the same xy plane. Then I used the 3D printing toolbox add-on I mentioned in a previous post here, I think it fixes some flipped normals or something. Then export as .stl and import that in the slicing program of your choice.
Let me know if you find a better approach for solidifying or if there are any bugs in the (quick and dirty) add-on!