Hankb - I hear you on the potential dangers of using such technique. Personally, I would recommend attaching the plunger handle to a wall, as opposed to a floor point to help mitigate that.
But that being said - it's not as dangerous as one might think.
As it turns out, 90% of the things we do here are dangerous, and a staggeringly large subset of activities of daily living are also dangerous. Take the operation of a chainsaw. Perfectly relevant if one wishes to remove a tree from his/her yard, but still potentially deadly.
Once you assume that sexuality is just another normal part of life, taking extremely minor risks to fulfill it seems no different than taking risks of the same magnitude to similarly address other aspects of our lives. EVERYTHING we do carries risk.
I agree - using a plunger attached to the ground is a risk that can be mitigated, but I think can still be done with a reasonable assumption of safety. I can think of better ways of addressing this (like removing the handle and manually stimulating that area), but I also understand the attraction inherent in this particular solution, and cannot fault the poster for suggesting it.
Like anything else fetish related, there is always risk. The key is understanding, and accepting, and actively working with that risk. If we didn't to that, than, well, I doubt any one of us would be here.