Currently the search feature has an option to search for closed proposals (ie: closed:1). This gives me closed proposals, but not in the sense that I first expected.
This combination of searches would probably not be used frequently. However, I'm sure that /search is used frequently. A more generic fix would be the addition of hot/progress/beta/launched/newest sorting in the search results as on the main site.
That said, I remember hearing that A51 is getting an update any day now (It's currently a hack onto the SE 1.0 engine), so this may already be in the SE 2.0 rev.