After re-reading realised that this is public facing and not internal so the security is a little less problematic.
By creating a new WLAN (ipad/visitor/sign-in whatever) and only allowing your ipad to use it you are leaving your existing patient/public network untouched.
At school I use MAC authentication a lot as our devices stay on premises and change rarely. Effective, "old school" and less secure but practical, it works and only devices I authorise/lay hands on can connect.