Draw on webcam canvas prior to AR.js processing it

[fingerskier]

I want to know if there is a feature, or mechanism, to draw on the canvas/image prior to it being analyzed by AR.js; My desire is to add draw a marker at an arbitrary location and still support RL markers. If there isn't: can you give some general guidelines for me to patch in this ability?

Samples I have seen show the arToolkitSource handling the webcam input behind-the-scenes. Is there an example which illustrates preprocessing the camera input before AR.js analyzes it?

Thank you!