I do not believe there are any restrictions on the use of strain relief devices in Classified Locations. I say this because... the NEC allows the use of "general-purpose" equipment in Division 2 locations when:
500.8(A)(3) Where specifically permitted in Articles 501 through 503, general-purpose equipment or equipment in general-purpose enclosures shall be permitted to be installed in Division 2 locations if the equipment does not constitute a source of ignition under normal operating conditions.
BTW, scope of UL 514b covers a wide variety of equipment. Some of the equipment has been tested for use in classified locations... but not all of it has been tested for this use.
Hope this helps.