If you can intercept an interstellar object to land something on it you can by definition also just launch the thing to the same trajectory.

Unless your payload is advanced enough to use the mass of the object for something (fuel, radiation shielding, construction materials, etc.) there is not much benefit in landing on an interstellar object if you just want to launch an interstellar probe.

Maybe actually if you wanted to send a really long range message it could help to put it on an interstellar asteroid - much more likely someone would notice a huge mountain flying through their system than a piano sized piece of metal, even if both hold the same message.