I'm surprised that you are being required to tie the instrumentation cable shields to a ground bus at the supply end.
My experience has been that on shielded low-voltage loops, fed from an ungrounded supply, you would bond the shields only at the instrument end, not at both ends. This eliminates the possibility of induced ground loops in the shield.
If the engineer absolutely insists that you bond at supply end to the isolated bus, then you should carry an insulated grounding conductor of the appropriate size for the upstream overcurrent protection back from the isolated bus and tie it to the UPS system ground.
Regards,
Brian
