Different routing for guest network

Redditers,

I have 150+ sites in the US with all kinds of connectivity back to my data center. Here are the main categories they fall in:

-MPLS + 4G (4G only for failover) -MPLS + Broadband with firewall(internet egresses out of broadband)

When a site gets broadband they are allowed to have guest wireless.

Today my MPLS is mostly single T1s. This year we'll be migrating to a new MPLS carrier and every site will get a minimum of 5M Ethernet. In the new design, no sites will have broadband. All internet traffic will traverse back to my data center.

With this upgrade, all sites will get guest wireless.
I would like to have the guest internet use a completely separate route for their internet access. So my corporate internet traffic will leave from NY, and my guest traffic leaves from California.

How can I have a separate route for this? I'll be doing bgp with my MPLS provider and will receive my default route from it (back to my DC). We use Cisco routers.

Ask any questions you need. Thanks in advance.