I don't know all of the particulars about BMST other than there's always unlimited free camping at the Bend in the Road - about 3 1/2 miles from Land's End (end of the pavement). There are absolutely no services out there, but there's a truck stop with showers and food, etc, about one mile away from the bend. Town is another 5 miles or so.
Perhaps someone will give better information on porta potties, but I know that at SCTA events we install about 6-10 at the bend in the road.
Cell service isn't an issue for me. I use Verizon and not only talk and text and surf, but also use the cellphone link to live stream the proceedings from SpeedWeek.