Is there a current regulation, directive, policy etc. that prohibits co-mingling of FMS assets with Army assets during overhaul or repair at organic depots?
I have reviewed AR 12-1 and 750-10 and did not find any guidance that forbids co-mingling; that is not to say that it is not contained in some other regulation.
That regulation is what I am looking for.
There is no regulation or policy directive that specifically says FMS items must be strictly segregated and cannot be used on US Army aircraft. The practice may have been an outgrowth of policies involving repair and return of FMS assets. Current policy in DoD 5105.38-M, para.C6.4.9 authorizes direct exchange (return of a reparable asset in exchange for a serviceable one) for non-end items. For end items, the same item must be returned and is lead time away.
Without doing extensive historical research, I know that at one time, a legal ruling determined that "direct exchange", or as it was called in the Air Force, Repair and Replace, did not have legal authority. The decision to strictly segregate FMS items may have been an outgrowth of this policy to ensure the Army did not run afoul of prohibitions on use of replacement parts for FMS assets.
Bottom line - there is a requirement to account for individual FMS end items returned for repair (the same serial numbered item must be returned to the customer that he sent in for repair). Cost of the repair must also be strictly accounted for, but there is no requirement that I am aware of to segregate FMS materiel, or prohibiting use of investment spares on US systems once they have been repaired and brought up to serviceable standards.