Yes you can add
/ip route add dst-address=XX.XX.XX.XX/21 type=unreachable
and then filter out in bgp filters all /24 routes.
Would it be possible for you to extend this feature to any kind of network interfaces using software instead of the hardware chips?This is supported on some RouterBOARDs that have appropriate switch chips that support port mirroring. For example RB450G and RB750