Thanks for your replies and links to other threads which I hadn't seen.
I think at the end of the day including album pre-release single track downloads or streaming previews under the release group for the relative album might be the best idea. That way at least we get an idea of the 'history' of the release itself, ie. seeing that a certian track or tracks were released prior to the full release.
This would be the case if the track is not to be considered a 'single'. We are obviously talking about digital download/streaming tracks only here of course.