Set Source

So, from my understanding, routes in the Linux kernel have can have a source associated with them.This is RTA_SRC attribute in netlink ( I think ). We basically want to ensure that when our routers get routes, locally generated traffic comes from the loopback address. Does anyone know a way to do this? I think if BIRD set RTA_SRC before sending the route to the kernel, it would work. Could this attribute be surfaced via BIRD easily?