When unsharp masking...or sharp masking for that matter... are done the contrast of the sandwhich,,,I am assuming a negative sandwiched with a positive..require one to use higher contrast paper to print the sandwich then they would without the mask, The usage of higher contrast paper will increase local contrast edge differentials which in turn would make the out of focus areas more rather than less prominent. If one were to sandwhich an negative with a negative mask and would one would be required to use a lower contrast paper to print the image than would otherwise be the case. The lower contrast paper would make less distinct out of focus as well as in focus areas edge differentials.
Of course how much effect the masking had on the viewer would be determined by the character of the out...or in... focus areas. Are those areas that are out of focus slightly soft or are those areas thouroughly blurred so that they are not even reconizable to the viewer.
Having said the above, I do not for myself believe that I would bother to use masking for purposes modinfying the out of focus areas. However, I have learned to avoid saying never. What seems to me to be not a very viable method for doing something may strke someone else as being very worthwhile for the application of time and effort.
While all of the above sounds convincing to me, I have never made comparison prints to check out such differences in out of focus characteristics by using and not using an unsharp mask. In other words while I am experienced with unsharp masking I have not tried to determine this particular effect.