I've thought of doing that with one of my MM escs to see how is works. If you use 3mm self tapping screws, they will go right between the fins of the sink and have very good holding power.

I don't think that you'll have to worry about a short, as long as it's only the heatsink that's touching the chassis. The MM heatsink is anodized all the way around and anodizing doesn't conduct electricity. Just make sure. Use a DMM to see if you can get a closed circuit on the sink with the leads. I've taken advantage of anodizing as an electrical insulator in MANY electronic projects and have never had any problems; but all of the parts that I used were type III anodized. I don't know if type II (like on the MM) is also non conductive.