I found a way to make this work (sorta)
My HTML page has an <img> element (positioned offscreen) and a canvas element positioned onscreen.
I used assignToHTMLImageElement to copy my QPixmap into the <img> element.
I then did canvas.getContext("2d").drawImage(img, 0,0) to copy the <img> into the canvas.
The result is a canvas that displays the QPixmap. But there also exists an offscreen copy in the <img> tag which seems "expensive" but it does get the job done and performance seems fine (I too needed to show upward of 30fps, and it works just fine.)
It does seem like the API should have a call "assignToHTMLCanvasElement" or, assignToHTMLImageElement should be fixed to allow the QPixmap to be assigned directly to a canvas element.