First thing would be to try looking at the IPTC data using another application other than PhotoMechanic to make sure the problem is with Aperture and not PhotoMechanic.
Preview should be able to do this but the best tool for this is ExifTool
Preview should be able to do this
In Preview you can see the field in the Inspector:
- From Peview's main menu bar: Tools > Show Inspector.
- Select the IPTC tab
I tested several images - when I set the IPTC Subject Code and exported the edited version as jpeg, the modification showed in Preview correctly in the IPTC Subject code field.
Tested in Aperture 3.5.1, MacOS X 10.9.1