Possible approach for small.net - ok, you know that big.net will drop any packets sourced from x.x.x.x if there's no route there (loose uRPF for downstream ISPs like small.net, strict uRPF for end-users.) So give them a route. Either give them a route on one of your direct interfaces to them, and then get rid of the packets by ACL or by null-routing it, or if that causes too much trouble, get yourself a 56kbps line from a spare router and advertise it from there.
