Bart -- I don't really mind a little vignetting, but if I do what you're suggesting and shoot a frame on film to calibrate for fall-off, etc., how would I use it to apply corrections to the scanned film frame?
In the (apparently) theoretical case that you do want to go through the trouble of shooting a 'featureless' LCC frame, you'd have to process that frame identically to your actual image (i.e. same camera/lens and fixed scanning settings). The basic idea about LCCs is that they are linear gamma images, just like the Raw sensor captures, and in linear gamma space it is almost trivial to balance out luminance differences. Once a Gamma adjustment has taken place, it all becomes much less accurate.
Could I import them both into Phocus or Capture One and then use the software's LCC correction function? Or does that only work with raw digital files? Or perhaps there's a layer/blending mode in Photoshop I could use for the correction?
For the best results, one needs to stay in a linear gamma colorspace (Raw luminance capture/scan, without gamma adjustment). Anything else becomes troublesome, so only software that's designed to handle LCCs or Flat-field calibration, should be used.