Speckworm, my experience is in S&T exchangers, I would normally proceed this way:
I would consider my scope to be the vessel, supports and anchor bolts. In my class of work the vessels themselves are pretty rugged and are not much of concern. Using my "favorite" seismic standard and treating the vessel as "equipment" rather than "structure" I would design the supports for the longitudinal load, anchor bolts for shear and check for overturning. I would normally consider the seismic load reacted by one of two supports and two of four anchor bolts as S&T exchangers normally have one end floating. It there were unusual features of the design they would be checked as well. If the customer specified a seismic standard it would be used. My scope stops at the baseplate, the foundation belongs to others.
It doesn't sound as though you are obligated to do anything, but you may want to consider the basics anyway.
Regards,
Mike