So, here is an idea: The victim is tied down to anchors in the bottom of a shallow pit, then covered in mud. The mud starts down by his/her feet and slowly flows up to engulf his head.
I did a proof of concept, in my new mud pit in the woods, today. Things did not go perfectly. My anchors weren't strong enough, and I had limited time.
I'm attaching some frames from the video. I'm curious if anyone likes this idea. I'm pretty sure I'll try it, again. I'm more likely to do it if there is some interest in the scene.