I've refinished several magazines, usually with duracoat over parkerizing or just parkerizing alone.
I've never done anything to the inside of a magazine except clean it. Totally ignoring the inside during the refinishing. Of course the insides were reparked, but without prep it didn't have any effect. Maybe darkened in some scratces a little.
So I've ignored the insides on purpose with no problem. However, I'm no expert, so I'm not sure this is the last word on the subject. My thinking was that I want the mag to function so I'm going to leave the inside alone. I just clean them and use them, and occasionally refinish the outside of one to match a new build.