An interface is configured to acquire IP address via DHCP. Option3 returned from the DHCP server is the default gateway.
An IP DHCP Pool is configured on the router with IMPORT ALL to pass clients the DNS servers included in the interface DHCP response. So we see that DHCP options can be dynamically passed to the client pool.
Here is the question, is there a way to build IP ROUTE 0.0.0.0 0.0.0.0 <DHCP OPTION 3 IP ADDRESS> ? The 851 routers will not have a dynamic routing protocol running. Prior to deployment to customer prem there is no way to know that the default static route will be, so it cannot be preconfigured.