This is the 2014 NEC:
(C) Structure as Equipment Grounding Conductor. Devices
listed and identified for grounding the metallic frames
of PV modules or other equipment shall be permitted to
bond the exposed metal surfaces or other equipment to
mounting structures. Metallic mounting structures, other
than building steel, used for grounding purposes shall be
identified as equipment-grounding conductors or shall have
identified bonding jumpers or devices connected between
the separate metallic sections and shall be bonded to the
grounding system.
This is 2017:
(A) Photovoltaic Module Mounting Systems and Devices.
Devices and systems used for mounting PV modules that are
also used for bonding module frames shall be listed, labeled,
and identified for bonding PV modules. Devices that mount
adjacent PV modules shall be permitted to bond adjacent
PV modules.
I think someone would be hard pressed to convince an AHJ that a mounting system is identified for use as an EGC without being listed under the 2014 NEC. Under 2017 listing is now required. Many people feel that “identified” means that the manufacturer just says it okay to use it but the NEC definition requires a higher standard. A third party evaluation is still required but it does not need to be done by listing, although listing is considered identifying. So think of it as listing lite.
Identified (as applied to equipment). Recognizable as
suitable for the specific purpose, function, use, environment,
application, and so forth, where described in a particular
Code requirement.
Informational Note: Some examples of ways to determine
suitability of equipment for a specific purpose, environment,
or application include investigations by a qualified
testing laboratory (listing and labeling), an inspection
agency, or other organizations concerned with product
evaluation.
So where does that leave you? Probably running a copper EGC to all the equipment and not depending on the rack.