Hostapd is probably how Cisco, Aerohive, and Ruckus are doing it. It's not that it's exceedingly rare it's just not everywhere as it's not part of wireless standards.
I'm only aware of patents related to certain systems which implement more than just the base idea of using more than one security key for a given SSID e.g. the Ruckus patent on how Dynamic PSK automatically creates and assigns multiple PSKs from a controller portal.
