re: Unfortunately...
Good point. Much of the original motion would have drifted into the random "Brownian motion" of the galaxy a long time ago, even if it could be detected. Spectral signatures can be tracked across much greater distances than proper motion.
Try this: If a group of stars were expanding from a common center, would they be moving faster (relative to us) than the general expansion of the universe? Could some common red shift be a signature?