Guild Backpacks are already allowable as Guild Mission rewards. Unlike accessories and banners, however, they are soulbind on acquire instead of account bound / soulbind on equip.
It would be nice if Guild Backpacks behaved the same way as the other gear you can get as Guild Mission rewards. Especially since backpacks are unsellable/unsalvagable since they’re backpieces. If you run on your most geared 80, chances are you’ll just have to junk it and that’s a tremendous waste.