A computer system (e.g., such as provided for a network service) operates to determine an inventory of time slots for a given airing. The computer system assigns a supplemental content item to a time slot of the given airing when the given airing is rendered on at least some of a plurality of playback devices. The computer system may structure a tag element to include data that is specific to a third-party, and then communicate a plurality of tags that include the tag elements to playback devices that are (or will) render the primary content of the airing.