I've already been through it all - the documentation doesn't cover it at all and from what it says it should work, but it doesn't. It's a bug in Steam and you need to get through to their technical team to get them to fix it. I'd happily talk to anyone on the Steam technical team, but I haven't been able to get through to them myself, even by filing bugs and doing everything I can think of. If anyone has any contacts at Steam, please try to get them on it!