Title pretty much explains it. I've tried rigging the poly object and routing it so that it makes a list and zl.filters out any notes that are at less than 1 velocity, but this has too many problems and isn't worth posting here.
This is what I want it to do: any time the object gets a midi note or set of midi notes, send it to the output as a list. Order doesn't matter. The list's size has to be the same as the number of notes currently held down. Velocity is disregarded, but it's probably useful for deciding when the notes are going to be on the list.