But I don't think this is designed to hit the same address via multiple upstreams. But to test the BGP links themselves.
I've used a 3rd party product for this from Noction called IRP. They probe the same addresses on all peers by having Filter based Forwarding setup for source address. The server tests each carrier via a different source and FBF insures the traffic leaves via the right carrier. They also keep a copy of the routing table by peering BGP internally to know what current choices are. This does also have the ability to inject routes to change both outbound and advertised routes.
you could use FBF with any other tool that does probing in a similar way.