Sorry, perhaps I misunderstand the routing request.
I thought you needed to use SOURCE based routing. When the source address is from these particular /30 then the next hop should be the AS200 router. This is what filter based forwarding does.
BGP can only inject DESTINATION routes into the peer relationship. And the peer will then forward to AS200 if the DESTINATION you inject is the target of the forwarding.
Is your need for SOURCE or DESTINATION based routes on AS300?
