Maybe you can get a case with the clip at the top of the case that would allow you enough motion to lift up the bottom so that the phone goes from vertical (hanging by your side) to now horizontal to the floor (face down).

It would depend on the case and whether or not tilting the phone from vertical to horizontal silences the ringer. Also how long it needs to be face down for matters; any longer than half a second and you would start to look very silly!

Probably unlikely to ever work since I think most cases have the belt clip in the middle. And yeah you would probably look ridiculous anyway :P