I don't think there would be any problem with unlocking the above if they were distributable through some stickied thread and then the game would just upload the filename of your skin, which would be used by others if they had it, otherwise, you'd just be the standard blue/red skin.
Anyone who wanted a skin could just go to said stickied thread, and could download them there. This prevents mandated periodic downloads that bog down servers and suck up bandwidth (unless the person so chooses). Whoever was in charge of distribution would probably create a standard pack, downloadable by others of the largest clans and highest rated skins that would be downloadable as one unit.
The same could be applied to trails, though modifying them could pose some issues as it could make it more difficult to see people at long range if you start messing around with them too much.
In regards to models however, I'm a bit more skeptical... Perhaps allow us to have the models, but don't actually let us put them ingame. Maybe just have contests once in a blue moon for model design and the winning model gets a spot in the game or something, even if its just for a limited time.
