I just discovered that you can buy plain old Backpack Straps from any Armorsmith (vendor)! I checked the wiki and there was no mention of it so it must be a recent change.
They're purely cosmetic but, hurray, now you can have a back piece on ever single char from Level 1.
I've updated the wiki too.
It appears they were added with the Feature Pack but I can't see it in the release notes.