Inject ethernet-based defaults to OSPF

[daniele]

Hello list,
I???m looking for some advice from someone that has more experience. I'm 
running bird 1.6.6.
I have two routers R1 and R2, each with a single internet uplink U1 and 
U2, running OSPF and sharing a LAN ethernet segment. Both U1 and U2 are 
dhcp-based ethernet segments themselves with a next-hop IP address that 
can change. I want R1 to be the primary LAN internet gateway and R2 the 
secondary. Should U1 fail, R1 has to route to R2 in order to reach 
internet through U2.
How would you recommend to inject U1 and U2 default routes into OSPF? I 
cannot use the next-hop ip address as it changes along time, and an 
interface-based static route would not work.

thanks a lot,
d