Separate Internet and Lan traffic over layer 2/OSPF

We have multiple sites with a 60c at each location acting as a router and a metro network, all connected and communicating over layer 2. Wan1 is on the OSPF and connected to the core router, which goes out to the internet and wan2 is going over a local internet connection as redundancy/dmz, in load balance mode, with a VPN to the core router as redundancy.

What we want to do is segregate the internet and local lan traffic that hits the main fortigate (100D) without creating vlans for our local network. Is it possible to do this on the WAN level?