Radiant uses picomodel, a library that supports a lot of model formats. q3map2 also uses that.
If your model format is supported, and Radiant enabled to load it in your gamepack config, then you will see the model in Radiant. Otherwise using static meshes for mapping would be kinda hard.
misc_model entity for loading static meshes can be added to the def file, or simply place and rename an info_notnull or something. Radiant even has a file selection dialog for static meshes. I don't think the QC needs to have misc_model at all, it is purely an editor / compiler thing IF you use Q3 format / q3map2.
misc_model for Quake 1 as in RMQ or Quoth works via QC and will confuse Radiant slightly, and q3map2 a lot. Keep the two well apart.
My blog has more recent shots of an entire staircase imported as a static mesh.
Static meshes will also work in Darkplaces because they are actually compiled into the BSP by q3map2. Darkplaces will never know what's a static mesh and what isn't. There are just polies. Again IF you use Q3 .map / .bsp format / q3map2. Quake 1 compilers cannot compile static objects into the map like that at all, hence why RMQ etc. load them via QC. No QC necessary in q3bsp.
The mesh display stuff in Radiant is for environmental objects, not for monsters etc.
Transparent clip: That would be hardcoded in bsp if it's Q1. For Quake 3 maps, the common shaders do exactly the same thing, so yes, when you map for Q3 format with a Q1 engine like in the tut I wrote, you have transparent clip etc. in editor via shader.