Painterly screen prints don't need to be simulated process.
I've been trying to figure out the easiest way to make my painterly work into screenprints, and I think I've settled on the best workflow for it. This's a quick and dirty proof of concept/client sketch video.
I paint each spot indepently on a spot-channel layer with 100% solidity in Photoshop. It allows me to change and tweak colors on the fly (very similar to working with spot layers in Manga Studio) and use a full range of value gradation on each layer.
With smart, conservative hue selection, one can make really intricate images that can be screenprinted just as easily as the lineart based spot-color illustration that most tees use these days.
The end result'll likely end up like these older movie posters which feature painterly, mass based imagery that was screen printed with a full value range for each hue.