Each service defined in the metabase includes a LogModuleList key at the /LM/<Service>/Info node. This key contains a comma-delimited list of logging modules available for that particular service. In order for your custom logging module to be available to a particular service, its name must be included in this list. You can use a script to add the name to the LogModuleList administration property.
For more information on using scripts to administer IIS, see Using IIS Admin Objects.