Set iTunes to display songs by album. That will let you identify the singles, since they'll be singletons while everything else will have multiple songs on the same album. Select all of 'em. Use Get Info-you'll get a "multiple items information" window. Assign the group of singles either a specific album title (i.e. "Singles") or alter the genre to something like "singles." Now a Smart Playlist will simply and easily grab all of these singles and collect them all together.
Yes, you do sort of have to figure out the "orphans," but my method makes it pretty easy to do.