I've published a globally reusable workflow which is associated with a parent or base content type.
Within one of my subsites I have a library full of documents that are child content types of this parent. My workflow does not appear as an available workflow for those documents. However if I (temporarily) permit documents of the parent content type to be added to the list then the workflow is available.
In SPD when I look at the child content types I see my workflow listed in the associated workflows box.
Should this work, am I missing something?