A method and system for controlling implementation of dynamic content modification. The disclosure provides for using at least one steganographically-encoded timestamp in a media stream transmitted to a media client as a basis to determine a transmission delay for media-stream transmission to the media client, and for providing the determined transmission delay as a basis to facilitate control over whether to have the media client implement dynamic content modification. In addition, the disclosure provides for receiving respectively from each of various media clients a report indicating transmission delay determined for the media client based on such steganographically-encoded timestamp data, and using the transmission delays for the media clients to establish a dynamic-content-modification footprint that could be used to control whether, where, and to what extent dynamic content modification will be applied.