You're thinking about it all from a 'implement it on the DOCSIS layer' perspective, which isn't the only way. They could just do what BT do, which is to just dial in a software PPP session from the router. The Superhub already has PPP capability and guest network capability, all they'd have to do is put together the right firmware (or demand Netgear do so), there's no hardware requirement for a different CPE.
There's no reason rate limiting has to be done on the CMTS side either. The original Netgear firmware is quite capable of that.
Plus since you're going to be dealing with captive portals and guest access it'd be far easier using the PPP solution and just plonking in a separate subscriber management system elsewhere than trying to integrate that all into the existing DOCSIS infrastructure. Indeed, there's plenty of free third party hardware and software that can already do all of the above, no need to make things hard for yourself
