RE: Controlling outbound traffic in a multihomed BGP environment

Routing based on source address is called "Policy Routing". IF you are on a cisco box, create an extended access-list specifying the source Ip's, and then match that access list in a route map to set the next hop. Apply the route map on ports facing that customer, building a chain from edge (facing the customer) to border (facing the internet.

Good Luck,