I am familiar with the techniques of film pre-exposure and print flashing, I use the latter regularly. Film pre-exposure I have used occasionally.
In both cases, the pre-exposure (or post-exposure, it really makes no difference), brings the material up to (and with film, even beyond) the threshold exposure. The difference is, with B&W film, this is the shadow area and with paper, the highlights.
In either of these areas, the extra blanket exposure will reduce the contrast in the less dense areas (again, shadows in film, highlights in prints).
Think of it arithmetically: 1 (arbitrary) exposure unit brings film up to Zone 1 density. Two units give us Zone II, 4 units Zone III, 8 units Zone IV, etc. Now, if I pre-expose with 1 exposure unit (Zone I density) and then give a normal exposure, my Zone I gets 2 units, my Zone II, 3 units (less than a doubling, hence less separation), Zone III 5 units (again less than a doubling), and so on. It is apparent that the lowest shadow zones have less contrast and separation. (A similar thing happens to highlights when flashing paper, but is less objectionable since the eye sees contrast easier in highlight areas.)
The problem with pre-exposing film is that most of us are striving for more separation in the shadows, not less. With print this is less of an issue, since it is the highlights which are affected and, as mentioned above, the eye does a better job of separating the highlights than the shadows.
In your situation, I would control contrast by reducing development, using compensating techniques and/or using SLIMTs to keep the highlights printable, i.e., classic Zone System contractions. I would add the extra exposure needed to place the shadows where I wanted them taking my development scheme into consideration, thus placing the shadow exposures well above the threshold. This would retain more separation and contrast in the shadow areas than pre-exposing the film. As I mentioned earlier, with color transparency material, pre-flashing is a viable tool, giving more exposure to the highlights while reducing highlight contrast somewhat, much like print flashing. For B&W negatives, however, I prefer as much contrast as I can get in the shadows, so I tend to avoid it.
My original question to Ralph has to do with why VC paper would not benefit from pre-exposing film, but graded paper would. I don't understand the reasoning yet... I'm sure Ralph will illuminate things for me.