html5 Canvas Mask with Alpha? How to use AlphaMaskFilter?

So I just discovered that we're not able to use alpha blending for elements being used as masks within a Flash CC Html5 Canvas project.

Knowing that the capability exists with cJS (ie AlphaMaskFilter), how would I go about implementing that with timeline script so that the generated code during export contains the right setup to make it happen?

According to the discussion at the link below, this should be possible:

http://community.createjs.com/discussions/easeljs/584-alphamapfilter-u sing-alpha-png-image-to-mask-jpg

What am I missing? Anyone cross this path yet?

Thanks!