I use the same workflow and it works pretty well. The most I could probably do with a plugin is associate slices with files in an assets folder that are continuously updated when changes are made. But you'd still have to drag the assets into Pixate every so often to update the layers in your project.
Seems like it would be fairly simple for Pixate to add the option to link layer assets to a folder so they're updated when the file is updated. That way a Sketch asset auto-generator would be much more helpful.