Strosser pin registration systems were what I used for making contrast masks. Ilford did publish a small manual on making contrast reducing masks with highlight seperators.
This proceedure was quite common in the 80's and I have read a few threads on this forum from other posters that do masking for black and white work.
Ilford introduced three levels of paper contrast for ciba . therefore the need for masks was limited. As well clients denied paying the fee to have a very good mask made.
The methods of making these masks are straightforward but do indeed require immaculate cleanliness for dust, as anything will show on a cibachrome and retouching the is very difficult if not impossible.
A good source of pin registration (used and cheap) would be any printing companies or film seperation houses in your local area or larger photolabs that did display work in the 80's and 90's as they would of done photocomp and would use the same systems ie strosser systems, nuark vacumn and light tables for registering multilayer films.