According to this site on artisitic cycling they refer to a trick called a "head-tube wheelie" which is probably a good name for it. BMX enthusiasts may have a different name for it, but that's essentially what it is. I was also able to find this other site referring to headtube wheelies. Also, this Youtube video refers to it as a headtube seated wheelie, and headtube backwheel riding.
After some Googling with the help of @dotjoe's answer, I've found that this trick is referred to a "Pedalling Death Truck" or a "Blender". There's also a trick called the "forwards spinning deathtruck" which is pretty much the same thing except the rider faces the other way around (at least as I understand it).