There have been many methods created for managing secret or covert
holdings, like the good-aligned temples in Thurazor/Five Peaks and Brad
Mournsinger's holdings in Kiergard. The main problem with all these
methods is that they require more work. Here is my simple alternative

Owning Covert Holdings:
Any holding of level (2) or less may be declared a covert holdings at no
cost. The holding can simply be marked with a superscript or
parenthasised "C" or "S." The holding in question generates 1 less GB
when rolling for collection or calculating law claims. Other than that
the holding may preform all normal actions. However, any contest
actions based out of the covert holding preformed against another regent
immediatly removes all benefits of covertness.

Attacking Covert Holdings:
If another regent wants to contest or physically attack a covert
holding, he must first succeed in an espionage attempt to discover its
location. The success is hindered by double the covert holding's level
below 2 (so a level 0 holding applies a hindering +4 to the espionage
attempt). If the action succeeds, the attacking regent may take
offesive action anytime within the next two months. After that time
another espionage attempt is required.