Zone Mortalis has been around much longer then Boarding Patrol. It's where they got the idea from!
As with anything wargame related, its popularity will differ from place to place. It suffers from needing a specific style of terrain to make it feasible, which historically has been verging on prohibitively expensive, as you had to get the terrain from
FW. Less so these days with many firms doing appropriate
MDF terrain, and what with 3d Printing being as prevalent as it is, it's made a lot more accessible. The Boarding Patrol boxes also made it easy to get into thanks to only needing two sets to fill out a 4x4 table.
Rules can be found in the Siege of Cthonia book. Alternatively they did appear in a white dwarf (EDIT-
WD#477, June '22)
about 18 months ago, so if you can find a copy of that issue or know someone who has it who could photocopy you the relevant pages.
There's a 3k Custodes army that's like 11 models or something. You could always go that route.