I would have to say, if "Supplimental Bonding" was required (supplimented by a Conductor, as the Hot & Cold are physically bonded at each mixing valve), it would be done for each unit - likely at the Water Heater its self, using a Conductor sized to the largest circuit value of that unit (largest OCPD in the unit's Panelboard).
If there may be issues where the Tenant's Feeder(s) could energize the Hot Water Pipes, then the Bonding Conductor would be sized according to the OCPD of the Tenant's Feeder.
This
_SHOULD_ fall under article 250-122, like an Equipment Grounding Conductor.
Fire away!!!
Scott
