I've found out that pdesolve doesn't discriminate regarding missing b.c.: it set the inlet dc/dz=0 when I had set an exit condition. That makes sense, but I wanted to cover all cases. The supplied missing b.c. is always the derivative=0.
My investigation has convinced me that pdesolve is supplying the missing b.c. I think it would be nice if the documentation mentioned this fact. The documentation in the help file is shown below. In actual operation, pdesolve doesn't allow one boundary to have both b.c. Also, it doesn't give an error when one b.c. is missing.
I suppose this is all meaningless unless a pdesolve replacement appears in Prime in the future.