Delta Creative Matte Exterior/Interior Varnish is a truly flat paint on sealer. I use it with my solid 1/6 heads that are fully repainted. I can see using it to spot paint eyes and parts of softer heads. It might have enough tooth to work as a base for pastels.
It's easy to find at most craft stores, if they don't have the small Delta bottles they might carry the larger bottles of sealers. They also make satin, gloss, and some texture finishes.