You could deploy your 512 on a dedicated vlan/subnet, put your 3825 in that same subnet using an available interface on that 3825 router, then you could use the external interface on the NME-502 to put the nme in that same subnet as the 502. When you start using the external interface, you have to shut down the internal interface on the nme.
Configure both the 512 and NME to use the router as default gateway on the new dedicated subnet. You would use L2 for redirection and ip forwarding for egress.
For a few weeks, I would monitor the wccp load balancing between the nme and 512 and if necessary, you may need to configure a weight so that the NME would get less traffic then the 512.
Joel Lamousnery CCIE R&S - 36768 Engineer, Customer Support Technical Services
CCIE R&S - 36768
Engineer, Customer Support