One thing I learned early in my military career is to befriend your supply sgt. If you had a good one, he was an excellent scrounger. 550 cord in any unit I served in was never in short supply-but, I was in the infantry, and we either tied or taped everything down (amply supplie of 100mph tape too). Most squads had their own rolls of 550 cord, as it served so many uses.
As for the APO addresses-I have never heard someone restrict that, but, it is unit specific. Likely, as Desperado said, to keep the hate mail to a minimum. Sad, but true.