Yeah, that was actually a question that had popped into my mind but I hadn't found the nerve to voice.
I don't imagine having gritty sand or small rocks between male and female flesh during intercouse would be pleasant for either party. On the other hand, I have little enough true experience with actual sex (yes you may laugh) to really judge.
A condom would only protect the guy from said filth as well, unless both a male and female condom were used, and that... not sure how well that could possibly work, to be honest.
And Nessie, you said "VERY few." Not "NONE." I can only guess this is a deliberate choice of words. Are there any --- such as the private, maintained pits --- that you think MIGHT be safe, or safer, or safe enough?
Breathplay is indeed another matter. But then I'm not suggesting just any gal/guy do it without any idea WTF they're doing. Like it or not there are many people who *DO* practice breathplay, consensually and with full knowledge of the implications, and who have the experience to know what's safely manageable and what's not... as well as what's pushing the edge and what's not. Also, there can be no denying that breath play is already part of any submersion scene like we've all seen in the videos Club MPV and the others have produced. Gals submerge all the time, and hold their breath only long enough to cut the camera to a different shot after they're out, to give the illusion they're still in there. While not quite the same I admit, even the use of hoses to create splattering air bubbles is breath play in a way. It's an illusion as you said, but the illusion is still that the gal --- or guy --- who went under is now suffocating below the surface.
The aforementioned scene of a gal "riding" a guy under could be managed easily enough with a camera cut and a hose also. She keeps humping the mud, someone off-screen blows into the hose to create bubbles right in front of her... voila. She's riding a guy down into his grave. Or heck, a second cut, and we see him emerge from beneath her once she's done... maybe she's the one that reaches below the surface for him after he "succeeded his mission." It's all good. I'm willing to accept illusions and good acting to some extent.
But I also have to say that not all of the illusions or acting are convincing. But we cannot expect too much I suppose, except for the best effort of the artists trying to produce the fare we enjoy. Which is why having true mud/qs affectionados doing the deeds I'd like to see would be that much more convincing. Because they're enjoying it also!
Note also that the M/F sex/violence/drama needn't be all in the pit itself. It would be amazing, but not necessary. Why can't a damsel threatened with a sinking death in quicksand try to "negotiate her release" before ever going in? That's very nearly straightforward porn as it is. Again to reference AcidTester's amazing works, the "blow me and I'll rescue you" scenarios don't necessarily involve mud in the woman's mouth... a quick cut from her at chin-depth, to him humping the mud when she "appears" to have gone under, would do the trick. Even if the actress submerges only just enough that we still see her hair at the surface in front of him as he humps, at that point since we can't see through the mud there doesn't really need to be fellatio going on. A little camera sleight-of hand (or sleight-of-other-body-parts

) can go a long way!
But add as much of the "real" stuff as you can visibly, and the rest is more convincing by default. Or am I wrong?
EDIT: Also, Nessie --- and all the other ladies on this site --- I don't want you thinking I'm a sadist in Real Life. I am NOT suggesting that the actresses --- or actors as the case could be --- really be put in deadly peril. I am NOT advocating RL rape in any form. I'm NOT saying a scene of sexual quicksand peril be done willy-nilly with no regard to the inherent dangers. I'm asking if it's possible to produce rougher fare of the type I fantasize about, and am hoping my arguments and suggestions give people food for thought and reason to push the envelope a bit. Yes my tastes are perverse and most "socially sane" women would consider me evil for them, but I'm sure I'm not the only person who would like to see this happen... male or female.