This vocabulary doesn’t seem to offer a suitable way. Nearby are:
This vocabulary doesn’t seem to offer a suitable way nor even similar/related properties.
There are so many vocabularies that it’s likely that something suitable exists, and if not, you could create a new vocabulary.
But these vocabularies don’t get recognized by the big search engines (they primarily support Schema.org, and in a few cases other popular vocabularies like OGP). This might change if they see widespread use, of course.
I think POWDER, which is superseding PICS and also standardized by the W3C, could be of interest:
POWDER — the Protocol for Web Description Resources — provides a mechanism to describe and discover Web resources and helps the users to make a decision whether a given resource is of interest. There are a variety of use cases: from providing a better means to describing Web resources and creating trustmarks to aiding content discovery, child protection and Semantic Web searches.