Be careful with dubbing anything big "interstellar medium". We reserve that term for the relatively unperturbed gas between heliospheres. But you guessed it, the interstellar medium is indeed focused by the sun: But that's a stark contrast with these rock or comet-like objects, which are almost indisputably ejected from a solar system sometime during or after the proto-planetary accretion disk stage. It's going to be impossible, right now at least, to demonstrate a relationship between the composition of these things and the makeup of the interstellar medium, because we have effectively a single point measurement of the interstellar medium across an entire galaxy. And remote sensing distant interstellar media is... well, I don't know how you'd do it. I'll think about that. It doesn't radiate, so you'd have to work with something passing through... Ohhh this is fun, thanks