Well, I probably should have done this to begin with. I pulled the head on the enlarger, and placed the negative against the bottom of the mixer. The diameter of the mixer chamber is smaller than the diagonal of the film. Therefore, the vignetting. It is physically impossible to avoid.
Now a question for those of you with different heads on your beseler 45mxt or equivalent. If you place a 4x5 negative against the bottom of the head (where the light comes out), does the negative fit completely inside the light source, or does it overlap the edges? (I'm trying to determine if it is worth buying, for example, a condenser head for my enlarger to do these kinds of enlarged, edge to edge prints.