I've also seen a new type of backpack that you can get through the Exchange or supply system. You won't find anything in MCO P1020.34F that specifically says you cannot wear backpacks. Backpacks are not in the Uniform Regulations and therefore are not authorized for wear as part of a regulation uniform. You will also notice that the same paragraph mentions organizational clothing, as described in paragraph 7000. Authorization to wear an Assault Pack is determined whether the Assault pack has been issued by the command or purchased commercially.
Organizational clothing has special considerations, and is discussed in detail below.
I can't find anything in the regulations that specifically addresses them. I've seen Marines for years wearing them, and I am pretty sure they are authorized, but I can't find the reference to back that up. Some chowhalls have even been allowing Marines to dine in coveralls. As explained below, organizational clothing is prescribed by the commander, so you will have to look to your local regulations for wear. However, coveralls cannot be prescribed as the uniform of the day.