Has anyone had any joy in seeing custom skins on line, for example if I put a custom skin can other online players see it?

Yes, or rather about 3 weeks ago when I was nosing around with how CoD client and server communicate (yeah, I know). It seems to 'dribble' the skins to each player, so can take tens of minutes to get going and sync up. It does cache them though. I'm not positive it works all the time either, i.e some skins seem to stall it.