To be totally honest here, I seriously doubt we'll make full texture replacement an option. With low poly models, the texture tends to inform a lot of the actual shape and detail of the object, so allowing that to be totally modified and redefined gives us some cause for concern (and that's not even going into the potential art style clashing).
I think at best we could possibly allow for alternate paint jobs (i.e. changing colors but not totally redefining the texture) - although even that might need to have some limitations placed on it.
Even if none of that is present though, I think the decal system is likely to provide a lot of room for customization.