I cannot find anywhere if gangplanks passive poison on-hit effect (which does magic damage) works with Rylai's Crystal Scepter.
I know this isn't viable in most instances, but I found myself wanting to get it as a last item in an ARAM game, but didn't because I assumed it didn't work. However it DOES work with brands passive (magic damage over-time as well), so now I am wondering if I was incorrect in thinking that it would not work.