I would like to open an new thread for this, since we are no longer on uploading, but on typesetting. First, I will answer this and let it stay here for a couple of days so everyone sees that it will move.
Laws outline the basics, but interpretation, which is where "threshold of originality" would be decided, is determined by case law -- the actual court cases and the decisions the judges make.
A judge would have to look at an edition or arrangement and decide how much originality went into it. There are widely varying approaches to this worldwide. Some countries have such a high threshold of originality that a very scholarly urtext edition wouldn't qualify for copyright because the editor put the piece back the way the composer had it in the first place, and that wouldn't be deemed original to the editor -- so he wouldn't qualify for copyright protection.
In some other countries, the opposite is the case -- the court apparently looks at how much time went into the edition, not necessarily how original the effort was. It would be in these countries that pure typesets with no alterations to the composer's original would probably be protected.
Finally, one of the reasons this is such a grey area is that most people won't spend hundreds of thousands testing this stuff in court. It's cheaper just to buy the edition.
Mark, to get down to the specifics you need, perhaps you could say why you are asking? Is it something you want to publish, or is it something you want to post at IMSLP? If the latter, the copyright reviewers can tell you what they will allow, and they will most probably only be interested in the PD status of your source, if you are uploading your new edition/typeset under a CC license. If it's the former, you might only need to look at what country you would be publishing in, and then ask if anyone here can tell you about the threshold of originality for that country.