A wise choice. I do not see a code compliant solution and the AHJ seems to have come up empty too.
His ideas are not compliant either. If you did find a route to run an EGC, you might as well run SER and be legal.
Where are the panels in relation to the outside walls?
The subs are in the center of the units so the new feed or EGC would need to be fished through the ceiling joist bay out to the back porch where it could be tied in. Running new feeds to each unit would get pretty crowded with conduits on the eave but a shared EGC would be much easier and inconspicuous, and IMHO would comply with 250.130(C) (1) & (2)
