Guest networks does not automatically allows guest's accecs to anything other than the in internet! Therefor you do not need a cable connection.
Pls se here: http://support.apple.com/kb/HT3477
"A Guest network is a second, separate network provided by your AirPort Extreme (3rd Generation or later), Time Capsule (2nd Generation or later), and AirPort Express 802.11n (2nd Generation). It allows guest network clients to access the Internet (and use Internet-related applications such as iChat and Safari)."
and ..." If you want to give guests access to network features besides the Internet (such as AirPort printers, Time Capsule disk(s), AirPlay, or AirPort disks associated with your primary network), consider using the Add Wireless Client feature. This will allow clients to join the primary AirPort network either for a day or longer without requiring the password. "