Is there any way of integrating a Cisco ISE Guest Portal with Meraki wireless in a manner which will secure the traffic (e.g. WPA2-PSK)? If this isn't possible at present, is it on a roadmap?
Have you considered switching to the Meraki hosted guest auth/splash setup? We use this quite successfully across a number of networks and deploy the guest SSID with a PSK and then present a sign in page.