I have a stack of screenshots to maintain, where what changes most often is the release information: so, I've made a separate .psd with the header and used File > Place to add it as a smart object. What I'm not having any luck with, is changing that single .psd file and seeing the update ripple through to all the screenshots. Tell me what I'm missing!
You seem to misunderstand Photoshop Smart Objects.
Smart Objects have no connection to sepearate files but are stored in the containing document.