It's been a long time since I did any Doom level editing but from what I can remember, you can actually distribute a pwad with just the graphics in it, and if you load the commercial doom.wad and then your pwad, it'll work just fine.
Now, as to the font being in the iwad, so you can pwad over it, I don't know. My gut tells me it's in the executable. I know all the text is in the executable, and I don't remember ever seeing the font images when browsing textures.