Second IP address same interface

Hi there!
I am giving PPPoE through mi LAN interface to my customers, by using private pools. So Masquerade and src-nat it’s active. Also, some others, are using public IPs from another pool, without masquerade.
And the ‘private’ customers, this way, are surfing by using the first WAN IP declared in the MT box.
But, the same WAN IP address is used by the MT to talk to the Radius Server, by administration, enforce RIP to the main router, etc.
So the question is how to tell the private ip pools -the customers in fact- to be natted by another IP ( the secondary one in the WAN interface ) so I can enforce some rules outside de MT box to allow/disallow traffic based on the WAN IP address?

Thanks

There is probably a better way, but you could add a bridge with only the wan interface in it. Then you assign one IP outside to the bridge and the other to the wan physical, so the interface and ip’s are tied to the nat.