Does anyone know of any open source color separations software I can feed an inkscape SVG into to get color separations for screen printing?

For colour separation you should be able to use the postscript driver of you printer and ask it to make the separations for you. I used that method for many years when colour separations were kind of a hassle to use . Would be a neat feature to have in Inkscape as well,.. to have a preview for that, using the postscript drivers. Maybe for the next Summer of Code?