Hi PipeHawk, if your interest is the Piping, and not the piperack, You haven't any problem, only is that your Desing Spectrum table,(Determined at the ground) Will be applied directly to the piping, without the "help" That the mass contribution of the piperack or other structures, can give to the dynamic event damping the efect and in consecuence the displacements, Mr. Ay opinion is Ok, The "accurate" for the support is wrong, but this is a "conservative" situation and not a omition input, but if your result are not ok, start to add these mass.
Regards