I think heimer's rockets apply these procs. Thats 3 (5 if ult is activated) procs at a very long range for a single target homing spell. I don't think his turrets apply procs when ult is activated, but someone should test it.
edit: If youre good at aiming, karthus would be very strong with this item since his q has a 1 second CD, and his ult hits every single champion once. Never again, someone will live with that 50 hp after requiem.