Rick, there are two ways that is this done that I know of:
i) Use a very fine "line" halftone screen for the void (rules, not halftone dots), with the background being lower screen ruling.
ii) The second easier method is, as you say, uses two regular halftone dot screens at different LPI for the same tint value. I can't recall if the screen angles are different, as well as the LPI.
Off the top of my head you will need to explore two general areas to achieve this. One is to use a raster EPS image for the void and to embed a custom halftone screen ruling into this image. Next your workflow would need to honour this custom halftone info and not strip it out and then be able to image the two different dots.
If you have not done this work before, it may become costly to produce, as you will have to get things correct on plate, then press and finally photocopy the end product to see if all works as intended.
I did my apprenticeship in security printing and we sometimes did this on the Purup proprietary prepress system that I used to use 20+ years ago - however I have not done this in the PostScript/PDF age...
Good luck and please keep us posted!
Stephen Marsh