So for what it's worth it looks like the new version 1.58 adds some features to the custom brushes, and includes some extra brushes in the "brushes.deck" file that basically do this but make it work - so you can use a brush that paints a halftone pattern.
It's not too hard to set up. If you drag the "brushes.deck" onto an open Decker window, it'll pop the Font/Deck Accessory Mover up. If you then add the "screenPrints" module to your deck, then that'll add some extra brushes to the toolbar that paint like half-tone patterns that you can use for dithering.