I hear people say all the time that, in the US, the Postal Service would be great for this, and I can't help but agree. Sure, they'd have to develop in-house expertise around these sorts of security systems (just as any new federal government agency put in charge of this would have to do), which could be difficult. But they have the ability to distribute forms, documentation, and tokens to pretty much everyone in the US, with physical locations nearly everywhere that can be used to reach those who don't have physical addresses.