Legislator released his GLT OPA Backpacks for All on the BI forums.

Quote Legislator:
While creating an mission having USMC force recons I noticed that they weren't able to carry the backpacks included in Operation Arrowhead. I was not satisfied with this situation so I had a look at the config values. Now here it is: the ability to use the Operation Arrowhead backpacks for every ArmA II unit! Please be aware that this mod doesn't add any new backpacks to certain units. It only adds a config value to the infantry baseclass so the units which are NOT included in Operation Arrowhead will have the ability to pick up and use the backpacks. Sadly the old models aren't capable of showing the backpacks, but of course they appear in the inventory.

Because of this config fix other community made soldiers will get the additional config value too. I've tested it with several other addons. PvP server admins should take of allowing / disallowing this addon because of the additional inventory space it may be considered cheating in matches. Coop players have less to worry. If you want to see this mod working, go to the editor, place a non-Arrowhead unit (USMC soldier for example) and several backpacks of Arrowhead. You will notice that you can add them to your inventory. It works with male civilians too! Sorry girls, but female inventories aren't capable of using the backpacks.