From playing Katarina a lot in the past I can say Trundle's Pillar interrupts her ultimate, at least if it pushes her. Of course, what doesn't? But that should indicate it interrupts channeled abilities, which point capturing for all intents and purposes is.
My best guess is it needs to noticeably push someone around to interrupt the capture though, so you'll more or less only get 1 person with it by placing it directly beneath them. Beyond that all sources of damage don't interrupt point captures, you can still capture points when Teemo and Malzahar place their DOT on you, so I don't see the 1 true damage helping out in that regard.
Beyond that, as cool as it is to interrupt channels by pushing someone 6 inches is, it is unimaginably annoying when you're on the recieving end. As a Katarina that waits for the opportune time to strike it's like really? REALLY?