Yes, the ADS provides the capability of Posting (on-stream and off-stream) within the WebSocket API by pushing content into a Cache located within theTREP. You can refer to the "Advanced" section within the documentation for an explanation. In addition, each example within the "Example" section provide Posting sample code.
This question was first asked in an Introduction to the Elektron WebSocket API webinar (29th of Nov). Please see here: Introduction to the Elektron WebSocket API