I know that this thread started with the caveat of no litho. But if one is attempting to gain the greatest impression of sharpness in a print then unsharp masking procedures are a definite boon to the impression of sharpness.
There are two factors involved with this impression of sharpness. The first is that there are edge effects that come into play. The second factor is that by using an unsharp mask the overall contrast of the camera negative is reduced. This reduction of the contrast level in the camera negative is offset by using a higher grade paper (either VC or graded paper). This increase in paper contrast brings into effect greater local contrast. This greater local contrast then brings about an impression of much greater sharpness into the print.
The procedure of unsharp masking is fairly simple and straightforward, as far as masking procedures are concerned. All that is required is some really inexpensive litho film and diffusion material.
Insofar as paper sharpness in and of itself, I think that in my experience, the effects of spectral diffusion are non apparent. One can not create any portion of the visual spectrum that does not already exist. In other words you can not create UV from either the visual or the IR spectrum. Todays papers have very well defined response sensitivities. So aside from possible flare (which is another matter in and of itself), it would seem that papers will only respond to the portion of the visual spectrum to which they are sensitive.