html2canvas has only basic text-shadow support:
// Not supported: text-shadow: -2px 0 #000, 0 2px #000, 2px 0 #000, 0 -2px #000;
// Just the basic X-offset, Y-offset, blur, color
text-shadow: 1px 2px 3px #555;
Some options:
Submit a pull request here: https://github.com/niklasvh/html2canvas/pulls
Position a CSS shadow styled html element over the canvas where you need it
Experiment drawing multiple abutting shadows on html canvas to see if you can create what you need.