Needing to make an image of this, I found a way. Here is my blog post on it: http://dominoherald.blogspot.com/2016/06/html5-canvas-to-png-via-rpc.html
in short, you can use the toDataURL method to get a base64 of the signature:
var canvas = document.getElementById(“view:id1:id2:canvasArea”);
var str = canvas.toDataURL(“image/png”);
then take that string and convert it to a file (note there is a 'header' in the returned string that needs to be removed).
A full XPage and the SSJS function I used it on my blog at the link above.