I'm resurrecting this two year old post because ... I'm Still Hoping For It.
I can understand that the developers don't want to have to store a bazzilion custom skins on their servers (if that's how it would work). But allowing a player to purchase the right to have one custom skin per mech could be a very popular option.
At a former job we played Quake3 at lunchtime, and I created custom character skins for my coworkers with their faces on them. It was a riot watching coworkers running around in the game. Imagine a CR-T with a photo of the player sitting at the controls on the windshield. At the moment the only customizations are the logos and colors/camo patterns that are (sorry to say) pretty blah. A few of the image skins are good (Crabby Scout for one), but anybody can buy them; none are going to be unique to the pilot.
So let's say you pay to add a skin. Later you decide to swap out for a different torso or pair of legs. You'll want to make a new skin to match, so you pay your money and you swap in your new skin. If the cost isn't too high people will want custom skins for all their mechs, and periodically change them. To keep offensiveness to a minimum have a rule of no curse words, raised middle fingers, copy-written logos, nude pictures, etc. Just skins like Houruck shows above. Allowing player-created skins can help generate additional interest. Look at what it's done for Team Fortress 2. Ten years on the market and still hugely popular, with enthusiast-made customizations added constantly. Something to think about...