If one is setting up unRAID for the first time and hasn't yet created an array, is it possible to use the Preclear plugin to preclear all one's disks simultaneously, or does this have to be done one at a time?

You can do several in parallel, but it's quite a resource hungry process, so be reasonable. I've done four preclears at the same time but I wouldn't be tempted to try any more than that, nor have I had the need.