I just got this working for my own customer – I apologize I don't have the exact settings - I'm not at the customer site right now.
In their case the Guest VLAN is completely isolated from production. The Motorala controller had interfaces on both the Guest VLAN as well as internal. Guests are directed to the Clearpass captive portal via the Internet - they go out the guest ISP and then back in via the corporate ISP and NAT into the captive portal.
Motorola was set up as a Radius client on clearpass using the internal address of the Motorola.
After much trial-and-error we found that we needed to configure the Motorola captive portal server mode to “centralized” (*not* "centralized controller"), set to the IP address of the Motorola’s Guest VLAN interface, and then set the captive portal URL (under the "web page") tab to point to Clearpass.
We also had to muck about a bit and create "welcome" and "terms" html pages which we uploaded to the Motorola, but we could have just as easily put them on Clearpass under content, I suppose.