I understand what you are wanting to do. I would mention, however, that preflashing of paper is inherently working against highlight tonal separation. It is by nature compressing highlight tonal values downward off the shoulder of the paper's curve into the upper midtone regions.
Originally Posted by glbeas
The other way to increase local contrast in the midtone and highlight tonal range and not exceed the paper's exposure scale is through unsharp masking. Since this is proportional to camera negative density, it tends to bring shadow density upward and tend to leave highlight density as it exists. The mask film base plus fog as it affects highlight density can be ignored in this instance since it is a constant throughout the mask.
An unsharp mask does not require extensive enlarger capabilities (such as a registration system). It can be produced very easily in the darkroom using ortho lith film and it produces repeatable results. Typical peak density on an unsharp mask will be in the .15-.35 range. This allows us to then print the mask-negative sandwich at a higher contrast range or paper grade then the negative itself would normally be printed. This has the effect of increasing local contrast while maintaining overall contrast.
I personally think that, considering human visual tendencies, we are accustomed to seeing more separation in highlights then in shadows. For that reason, when we view a print, the effects of shadow compression are less troublesome then compression of the highlight values through preflashing of the paper. However, I would go on to say, that no blanket assessment can be made in all cases. Since each image must be evaluated on it's own merits.
There is no "free ride" in any of the these manipulations of the paper exposure scale. Since that would indicate an ability to depart from the laws of sensitometry. We can, however, affect the scale so long as the overall scale is not exceeded.